








































































































































































































LIBRARY OF CONGRESS, 

H E- ° i X - 5 - 

Chap,Copyright No.__ 

Shel f. 3 . 

_US© 1 

UNITED STATES OF AMERICA. 



















* 




Copyright, 1897, by B. Thorpe, 
Publisher of The Keystone. 



r' 





X 





4 

The Optician’s Manual 


v 



✓ 




CONTENTS, 


Chapter 1. 

Introductory . 

Origin and History of Eye-helps. Optical Science as a Study. 
Widening Field for Optical Research and Practice. 

Chapter 2. 

The Eye Anatomically . 

Its Component Parts. How it is Protected. Its Muscular 
Control and Precision of Movement. 

Chapter 3. 

‘Optics . 

Theory of Light. Reflection from Plane and Curved Sur¬ 
faces. Refraction and Angle of Incidence. Lenses in Rela¬ 
tion to Light. Foci. Formation of Images. 

Chapter 4 . 

Tenses . 

Material of Which Lenses are Made. Varieties of Lenses. 
Action of Prisms. Decentered Lenses. Optical Center of a 
Lens. Mounting of Lenses. Spectacle ’Frames and Parts 
Thereof. Chromatic and Spherical Aberration. How to 
Measure and Test Lenses. 

Chapter 5. 

Numbering of Lenses .... 

Old Systems of Numbering. Dioptric System. Metric and 
Inch Systems Compared. Advantages of the Metrical System. 

Chapter 6. 

'The Eye Optically .. 

Physiology of Vision. Refracting Media of the Eye. The 
Retina, its Yellow and Blind Spots. Field of Vision. Ac¬ 
commodation. Correction of Optical Defects. Apparent 
Position of Objects. Size, Movement, and Color Sensations. 
Theory of Single and Double Vision. 









6 


CONTENTS. 


Chapter 7. 

The Use and Value of Glasses. 145 

The Public and Glass-wearing. Optical Defects Requiring 
the Use of Glasses. Various Kinds of Glasses—Corrective, 
Protective, Colored, etc. 


Chapter 8. 

Outfit Required ... 15^ 

Optical Education. Books of Reference. Case of Test- 
lenses. Test-types. Metric Rule. Record-book. Ophthalmo¬ 
scope. Retinoscope. Prisoptometer. Ophthalmometer. Kera- 
toscope. Phorometer. Optical Bracket. Perimeter. 

Chapter 9. 

Method of Examination.. 183, 

Preliminaries to Examination Proper. Points to be Noted. 

Use of Pupilometer. Illumination of the Eye, and the Exami¬ 
nation of Cornea, Aqueous Humor, Iris, Pupil, etc. Acute¬ 
ness of Vision Defined. Use of the Ophthalmoscope in 
Direct and Indirect Examination. Use of the Retinoscope. 
Shadow Test. Pin-hole Disk. Test-types of Snellen, and 
Their Use. Testing for Hypermetropia, Myopia and Astig¬ 
matism. Testing for Accomipodation. Testing for Prisms. 
Muscular Anomalies and Their Detection. Dot and Line 
Test. Maddox Test. 


Chapter 10. 

Presbyopia .. 295 

Accommodative Power of the Eye as Affected by Advancing 
Years. The Effect of Age on the Crystalline Lens. Presby¬ 
opia Defined. Its Beginning, Symptoms and Diagnosis. 
Presbyopia and Refractive Power at Different Ages. Presby¬ 
opia in Relation to or When Complicated with Hypermetro¬ 
pia, Myopia and Astigmatism. Glaucoma, its Symptoms and 
Diagnosis. Intra-ocular Tension. Glasses that Correct Pres¬ 
byopia and the Rule for their Selection. Tables of Presby¬ 
opia. Amblyopia. Fitting Frames Orthoscopic Lenses. 
General Remarks. >, 






PREFACE TO FIRST EDITION. 


T N presenting this volume to opticians, we are happy in the 
knowledge that it requires neither introduction nor eulogy. 
The publication of The Optician’s Manual was begun in 
The Keystone of May, 1890, and an installment has appeared 
in every issue since that date. Every chapter of the Manual 
thus far published has, consequently, been already read by a 
majority of the opticians of the United States, and their ap¬ 
preciation of its worth may be inferred from their impatient 
demand for its republication in book form. This volume was 
compiled in response to this demand. To the subject-matter 
it is unnecessary for us to refer. Its simplicity of style, per¬ 
spicuity of language, system and sequence in compilation, ab¬ 
sence of technical terms, and the reliability and practical char¬ 
acter of its wealth of information have already been the subject 
of general eulogistic comment. We present it to the trade in 
response to their own solicitation, and rejoicing in the assur¬ 
ance, beforehand, that we have made a valuable contribution 
to the optical literature of the time. 

We feel we have greatly enhanced the value of the 
Manual, as a book of ready reference, by the copious Index 
appended thereto. The reader can turn to this Index with the 
expectation and satisfaction of finding, at once, any and every 
point and subdivision of a subject contained in this work. 




PREFACE TO SECOND EDITION. 


HT HE entire first edition of The Optician’s Manual was 
sold within six months from the date of publication, a 
record of sales equaled by few works of a scientific character. 
The value of the book and the need for it are thus conclusively 
established. It has received the unqualified endorsement of 
•optical teachers, who have recommended it to their pupils, and 
of practising opticians, who have confessed their indebtedness 
to it for information found in no other work on the subject. 

Appreciating the responsibility resting on the publishers 
of such a work, we have taken advantage of the opportunity 
afforded us by the publication of a second edition to revise the 
entire volume, make ’ valuable additions to several chapters, 
and increase the number of explanatory diagrams and illustra¬ 
tions. These additional illustrations will be found a specially 
valuable feature. The Manual, in its present form, is con¬ 
ceded to be the most practical work on optical science, valu¬ 
able alike as a text-book for the student and a work of refer¬ 
ence for the practising optician. 



















































































. 













































































































1 































Prof. Donders 


LLUSTRATION FROM TIFFANY’S ANOMALIES OF REFRACTION, 
BY THE HUDSON-KIMBERLY PUBLISHING CO., KANSAS CITY, MO. 



















Prof. F. C. Donders, 

To whom the optical world concedes the distinction of being 
the father of practical ophthalmology, was born on May 27,. 
1818, in Tilbury, Holland. He was the ninth child and first 
son of poor parents, and while yet in his infancy lost his 
father. His solicitous mother, however, contrived to procure 
for him facilities for education, and such was the precocity of 
this remarkable boy that ere he reached his teens he had so 
mastered the Latin language that he was earning his living as 
a sub-teacher in a classical school. He continued his studies 
of Latin until his seventeenth year, and his mastery of this 
useful language served him well in his subsequent professional 
career. He began his medical studies in the military school 
of Utrecht, and from the first day of his student-career he 
manifested that industry and aptitude in scientific research 
which were destined to make his name immortal. When 
twenty-two years old he occupied the chair of military surgery 
at La Haye, and two years later he was professor of anatomy 
and physiology at the same medical school in Utrecht at 
which he began his medical studies. In 1889 he died in this 
same city of Utrecht, after a career of successful research that 
entitles him to the lasting gratitude of humanity and to a 
niche in the temple of fame, side by side with the greatest 
luminaries of this great century. 

While Donders did not limit his wonderful intellect and 
tireless industry to any single subject or science, the optical 
world is particularly indebted to him for his achievements in 
ophthalmology. His researches in hypermetropia and astig¬ 
matism gave to ophthalmology a new and wider meaning, 
practically developing it from an experimental into an exact 
science. His principal optical works are “Study of the 
Movements of the Eye,” “Astigmatism,” “Anomalies of Ac¬ 
commodation and Refraction of the Eye.” There are other 
and great men who owe immortality to their valuable re¬ 
searches in ophthalmology, but none who more deserves 
the gratitude of humanity than Professor F. C. Donders. 


11 
























; 










, 





































































































































































































































CHAPTER I. 


INTRODUCTORY. 


The first use of spectacles was, probably, for the correc¬ 
tion of presbyopia, or old sight, about six hundred years ago; 
and to Roger Bacon is generally given the credit of the first 
knowledge of their use. 

For many years after the discovery of spectacles, their use 
was confined to supplying the deficiencies of the eye conse¬ 
quent on age, and no special advancement in their use was- 
made. During the past half century, however, the subject has- 
been carefully studied by eminent specialists, and the greatest 
advances have been made in the treatment of the eye and its 
diseases, and the correction of the various optical defects by 
properly adjusted glasses. 

Any one who reads the history of Ophthalmology, and 
compares the past with the present, is forced to the conclusion 
that the advancements that have been made are little short of 
marvelous. Fifty years ago the whole subject was shrouded 
in mystery and uncertainty. Hypermetropia was not known 
as the factor in the causation of so much headache, eye-ache 
and neuralgia, and of so many cases of blurred sight and irri¬ 
table eyes; these cases were not understood, and hence were 
looked on as incurable, or classed under the general head of 
amblyopia. Myopia was recognized by its subjective symp¬ 
toms, but was not known to depend on a lengthened eye-ball 
caused by a diseased condition of the fundus of the ball. As¬ 
tigmatism had been heard of, but its true significance, was far 
from being understood. Strabismus and diplopia, in all their 
varieties, were recognized by their objective and subjective 
symptoms, but the theory of binocular vision, the action of the 
ocular muscles, and the possibilities of the benefit to be derived, 
from prisms, were as yet undiscovered truths. 

13 




34 


INTRODUCTORY. 


Perhaps in no other branch of science has such progress 
been made, so much valuable relief to suffering humanity been 
furnished, as in this science of Opthalmology; and the ad¬ 
vancements made have all been in the direction of simplicity, 
.as explained by the well-understood physical laws of optics, 
which account for so much that was formerly obscure and un¬ 
intelligible. To the beginner, the study of optics seems to 
present many difficulties, and to be surrounded with almost 
unsurmountable obstacles. But these difficulties and obsta¬ 
cles are more imaginary than real, and as rapidly disappear 
before a zealous student’s application as the morning dew 
before the sun. 

The subject may, for simplicity’s sake, be said to be com¬ 
prehended under three heads: 

First: A knowledge of the eye anatomically and optic¬ 
ally considered, which includes the normal and the ametropic 
eye, the various defects to which it is liable, and the proper 

adjustment of glasses for their correction and relief. 

Second: A knowledge of some of the simpler laws of 
optics. 

Third: A practical knowledge of lenses (simple and 
compound), their action on rays of light, and the laws that 
govern their adjustment. 

The reader will see from this that a little time well applied 
in study and thought will open up to him a large, fruitful and 
well-cultivated field, covered with much fruit of scientific and 
practical knowledge, which he can have for the picking; and 
hence all are urged to commence the study of the subject with 
confidence in their ability to thoroughly master it and to make 
practical use of it in their every-day business life. 

No age is now exempt from the use of glasses. They are 
placed on a child that is little more than able to talk, as well as 
on the aged patriarch who finds them indispensable to his hap¬ 
piness and comfort. Indeed, it is an indisputable fact that 
every person who lives to reach the age of fifty or more re¬ 
quires glasses at some period of his life, either for reading or 
distance. To many persons they are an absolute necessity at 
all ages, while those persons whose eyes are perfectly emme- 


INTRODUCTORY. 


15 


tropic require them for reading in middle life, on account of 
the changes wrought in their eyes by age. 

Those rare cases we sometimes see or read of, where per¬ 
sons have never worn glasses, and are able to read all their life 
without them, are but the exceptions that prove the rule; they 
are almost invariably myopic, but are not conscious of it; or, if 
they are, they refuse from mistaken notions to wear glasses, 
preferring to sacrifice their distant vision. It is the myopic 
condition of the eyes that enables them to read without glasses; 
but the fact remains that they require glasses to make their 
sight perfect for distance. 

The affections of refraction and accommodation of the 
eye are constantly assuming more and more importance, and 
are engaging more and more the best thought of the most 
skilful ophthalmologists. For it is now known, indeed the ex¬ 
tensive use of glasses has proven, that a large and ever-increas¬ 
ing class of eye-troubles (as, for instance, some forms of 
asthenopia and amblyopia), which were formerly considered 
incurable by any remedial measures, are not due, as was sup¬ 
posed, to organic change and disease of the structures of the 
eye-ball, but are, in reality, dependent upon some anomaly 
of the refraction or accommodation, and hence are now found 
to be readily amenable to glasses; and diseases that heretofore 
could not be checked by any of the means known to the physi¬ 
cian, are now promptly arrested in their progress and cured 
by the adjustment of the proper lenses. 

Although the use of glasses was at first due to an acci¬ 
dental discovery, their adjustment is now placed on a higher 
plane, which is controlled by the unvarying laws of mathe¬ 
matics. 

Persons with weak eyes, and some who were never con¬ 
scious of any defect in their sight, are enabled by a careful 
adjustment of glasses to see in a manner they never before 
thought possible. Others, who were compelled to abandon 
their chosen callings on account of failing sight, are sent by 
the oculist or optician back to work with eyes practically as 
good, and in some cases better, than ever. Even cross-eyes, 
sore eyes, and some affections of the lids can be cured by the 
proper adaptation of glasses. 


i6 


INTRODUCTORY. 


In view, then, of the great value of glasses to every in¬ 
dividual at some period of his life, and of their absolute neces¬ 
sity to many others at all ages, it is high time that all prejudice 
against their use should be done away. 

The greater the advances that have been made in the in¬ 
vestigation of the affections of refraction and accommodation, 
the more evident it has become how very necessary it is that 
they should be thoroughly and carefully studied and scien¬ 
tifically treated. Opticians will, therefore, realize the neces¬ 
sity of making themselves perfectly familiar with just such 
knowledge as is presented in this volume, always keeping in 
mind the fact that it is only by applying the knowledge so 
gained to the practical examination of a large number of cases, 
that he can expect to acquire the requisite facility and experi¬ 
ence necessary to satisfactorily and accurately adjust glasses 
for the correction of the various disorders of refraction and 
accommodation as met with in daily practice. 


CHAPTER II. 


THE EYE ANATOMICALLY. 


Every one must admit that an organ which is so necessary 
to our usefulness and happiness as the eye, cannot be unworthy 
of our serious attention; indeed, the intelligent care which 
should be given to such an important member of the body 
requires some knowledge of its structure and function. Even 
apart from this it would be natural to suppose that a subject so 
interesting and important would surely attract the attention of 
every educated mind; but, alas, the universal testimony of all 
those observers who have the best opportunities for ascertain¬ 
ing the true facts, is that such is not the case, but that the peo¬ 
ple generally know almost nothing about the structure of the 
eye or the care of the sight. 

In these days the intelligent optician is expected to know 
pretty nearly everything about the eye, and on account of the 
prevailing popular ignorance on the subject, he will be asked 
all sorts of questions about it, and he will be constantly looked 
to for advice when the eye or sight becomes affected in any 
way. If the optician is able to meet any reasonable require¬ 
ment in this direction, he will inspire his patrons with confi¬ 
dence in his ability to successfully fit them with glasses, and 
thus add much to his reputation as a skilful optician. 

The limits of this work, however, permit but scarcely more 
than a brief outline of this branch of the subject, just sufficient 
to afford an intelligent idea of the anatomy and physiology of 
this wonderful organ, and to lay a sufficient foundation upon 
which the more practical branches, which are to follow, can be 
securely rested. 

The eye-ball is nearly spherical in shape and measures 
about an inch in diameter. A glance at the diagram will show 

17 




18 


THE EYE ANATOMICALLY. 


that there is a segment of a smaller sphere engrafted upon the 
anterior portion of the larger sphere, and, consequently, if the 
antero-posterior diameter (that is, from before backward) is 
one inch, the transverse diameter would be about the twelfth 
of an inch less. The diagram also shows that the larger 
sphere forms about five-sixths of the globe, and the remaining 
sixth is made up by the segment of the smaller sphere. 

For convenience of description, the eye is regarded as 
consisting of three humors contained within three membranes. 

The humors, counting from before backward, are: 

1. The Aqueous Humor. 

2. The Crystalline Lens. 

3. The Vitreous Humor. 

The membranes, counting from without inward, are: 

1. Sclerotic and Cornea. 

2. Choroid, Iris, and Ciliary Processes. 

3. Retina. 

THE SCLEROTIC. 

The sclerotic is the external coat; it is a tough fibrous 
membrane, having almost the resistance of leather, and is 
about of an inch thick. It is the skeleton or framework 
of the eye, without which its shape could not be maintained. 

The sclerotic is white and glistening in appearance, and 
is popularly known as “the white of the eye.” It furnishes 
attachments for the external muscles that move the eye-ball, 
and indeed all the tissues, membranes, muscles, etc., are at¬ 
tached to it, either directly or indirectly, or dependent upon it 
for support. It practically surrounds the eye-ball, covering 
the larger sphere, or the posterior five-sixths of the globe of 
the eye. 

The sclerotic is pierced behind by the optic nerve, a little 
to its inner or nasal side. It is much thicker behind than in 
front, where, as it thins out, it passes (with some changes in its 
structure) into the cornea, so that the entire external mem¬ 
brane of the eye is continuous as a single membrane. The 
sclerotic is an opaque membrane, but as it passes into the 


THE EYE ANATOMICALLY. 


19 


cornea it loses these elements which render it opaque, and 
becomes colorless and transparent. 




a 


Figure of the Eye. 


A The Crystalline Len*. 

B The Aqueous Humor. 

C The Vitreous Humor. 
a a The Canal of Schlemm. 

b b The Iris, the opening between being the Pupil. 

k The Capsule of the Lens 
// The Epithelial Covering of Cornea 
mn The insertion of the Muscles in the Sclerotic. 


I) The Cornea. 

E The Retina. 

F The Yellow Spot. 
d The Optic Nerve. 
h h The Ciliary Body. 
g g The Choroid Coat. 
e The Zone of Zinn. 


THE CORNEA. 

The cornea is the projecting transparent portion of the 
external coat. It is joined to the sclerotic very much like 
a watch crystal is set in its case. Its degree of curvature 












20 


THE EYE ANATOMICALLY. 


varies in different individuals, and in the same individual at 
different periods of life, it being more prominent in youth than 
later in life, when it gradually flattens. 

On account of its transparency we look directly through 
it and see only the colored iris and black pupil behind it, and 
hence it is best seen by looking at it from the side or by re¬ 
flected light. Although colorless and transparent, the cornea 
is tough and unyielding, and thus supplements the sclerotic 
membrane in the protection of the contents of the eye-ball. 

The cornea is the window of the eye, “through which the 
individual looks out into the world”; and as the window-pane 
should be of good quality and cleaned from dirt, in order to 
afford a distinct view of objects, so care must be taken to pre¬ 
serve the brilliancy and transparency of the cornea. If the 
relative position of the cells composing the cornea be altered 
by pressure from within or without, it becomes steamed and 
cloudy, as also happens in injury or inflammation of this 
structure, when its usefulness is correspondingly diminished. 
No matter if every other part of the eye be normal, with an 
impaired cornea perfect vision is impossible. 

One peculiarity of the cornea is that it contains no blood-¬ 
vessels, all of them terminating in loops at its circumference. 

CHOROID. 

The next or middle coat is the choroid, which invests the 
posterior five-sixths of the globe of the eye, and forms a 
lining for the inner surface of the sclerotic. 

This is the vascular and pigmentary coat of the eye-ball, 
that is, it contains most of the blood-vessels of the eye; in fact, 
it seems to consist principally of a network of blood-vessels,, 
lined with a layer of flat, dark brown or black pigment cells. 
The blood-vessels supply nutriment to the various parts of the 
eye, while the use of the dark surface is to absorb the excess 
of light, which would otherwise dazzle and prevent accurate 
vision. 

The choroid, like the sclerotic, is pierced behind by the 
optic nerve. As it approaches the front part of the eye, it 
folds upon itself and forms a series of folds or plaitings, which 


THE EYE ANATOMICALLY. 


21 


are known as the ciliary processes, which are arranged in a 
circle behind the iris and around the margin of the crystalline 
lens, and they gradually merge into what is known as the 
ciliary muscle, or the muscle of accommodation. 

THE IRIS. 

The word iris means a rainbow, and it receives this name 
from its various colors in different individuals. It is this struc¬ 
ture which gives to the eye its special color, and upon which 
a large part of its beauty depends. When we speak of a blue, 
black or brown eye we mean the color of the iris of that par¬ 
ticular eye, and it is usually in accord with the general coloring 
of the individual; blondes generally having blue and gray 
eyes, and brunettes brown or black eyes. 

It is an interesting fact that the eyes of new-born babes 
are always blue, and they do not begin to assume their per¬ 
manent color until the sixth or eighth week of life, being then 
formed by the addition of a greater or less amount of dark 
pigment. 

The eyes of albinos are pink, not from this color of the 
iris, but from the reflection seen through it of the red blood 
in the vessels of the choroid, in which membrane there is also 
a lack of pigment. The sight of such an eye is always defi¬ 
cient, and they are painfully sensitive to light, against an excess 
of which they have not the natural protection of a darkly 
pigmented choroid. 

The iris is a thin, circular shaped, contractible membrane, 
suspended in the aqueous humor behind the cornea and in 
front of the lens. It is perforated slightly to the nasal side of 
its center by a circular aperture, the pupil , for the transmission 
of light, thus forming a curtain stretched across the interior of 
the eye. There is a popular notion that dark eyes are 
stronger than light ones; the only foundation for this idea is 
the fact that they are better protected against excessive light. 
Hence light eyes prevail among northern nations, and dark 
eyes among the races who live in the glare of a tropical sun. 

The muscular system of the iris is involuntary, that is, 
it is not under the control of the will, and hence we are not 


22 


THE EYE ANATOMICALLY. 


able to change the size of the pupil by the strongest effort of 
our volition. 

The muscles of the iris consist of circular and radiating 
fibres. The former surround the margin of the pupil on the 
posterior surface, forming a narrow band about the thirtieth of 
an inch in width; these are the fibres that contract the pupil. 
The radiating fibres converge from the circumference toward 
the center, where they blend with the circular fibres, and by 
their action enlarge or dilate the pupil. 

Through the action of these two sets of muscles, the 
pupil has the property of changing its size, thus regulating the 
amount of light admitted to the retina. When we pass out 
into the bright sunshine of mid-day, the pupil immediately 
and instinctively contracts, to protect the eye from the irritat¬ 
ing glare of a flood of light; and when we return to a darkened 
room, it dilates to admit as much as possible of the insufficient 
light. 

Belladonna, or its active principle atropia, if applied to 
the eye or taken to excess internally, dilates the pupil widely; 
while eserine and pilocarpine contract it. Opium also has the 
effect of contracting the pupil, and this is one of the first 
symptoms looked for when opium poisoning is suspected. 
When we are not looking at anything closely, or when we look 
at a distance, the pupils dilate, as is also the case in meditation. 
The pupil grows smaller with the advance of years, and by 
shutting out diffusion circles partially compensates for the 
impaired vision of old age. 

The pupil appears black, because of the lack of intra¬ 
ocular illumination, for the same reason that a small opening 
into a dark closet is black. The pupil can be illuminated and 
made to appear of a bright red color by the reflected light 
from the mirror of an ophthalmoscope, just as the closet can 
be lighted by a candle held near its door. 

The ciliary muscle consists of involuntary fibres, and con¬ 
sequently is not under the control of the will; it acts automati¬ 
cally on the approach of objects to, and their recession from, 
the eye, just as the muscular fibres of the iris when exposed to 
light or when shaded from it, and hence the ciliary muscle 
plays an important part in the accommodation of the eye. It 


THE EYE ANATOMICALLY. 


23 


is a grayish circular band about an eighth of an inch broad,, on 
the outer surface of the fore part of the choroid. 

THE RETINA. 

The internal or nervous coat of the eye-ball is called the 
retina, and it is the most important membrane of all; indeed 
all the other structures of the eye may be considered sub¬ 
servient to this one, as on it are formed the images of external 
objects by means of which we are said to see them. It is a 
delicate nervous membrane upon the surface of which the 
images of external objects are received. 

The retina is continuous with the optic nerve, in fact it 
seems to be the spreading out of the nerve, which pierces the 
sclerotic and choroid to form this membrane, and by means of 
which the impressions are carried to the brain, and hence the 
eye and the brain are in the most direct and constant com¬ 
munication. 

In order that the retina may properly perform its func¬ 
tion it is necessary that it should be in a healthy condition, 
and, if not, the sight is correspondingly impaired. The retina 
may be compared to the paper on the walls of a room, and 
like wall-paper it sometimes becomes loosened, as in “detach¬ 
ment of the retina,” which is apt to occur in cases of extreme 
myopia and results in serious impairment of vision and even 
total loss of sight. 

The retina corresponds to the delicate film of the pho¬ 
tographer’s camera, which receives the image of an object 
and allows it to be stamped upon its surface. There is one 
great point of difference however; while the film of the camera 
is confined to one impression, the capacity of the retina is un¬ 
limited. The image of one object after another is formed on 
the retina, as quickly as the eye can move and change its 
point of sight. 

In the center of the posterior part of the retina, at a point 
corresponding to the axis of the eye, in which the sense of 
vision is most perfect, is a round, elevated, yellowish spot, 
having a central depression at its summit. The retina in the 
vicinity of this yellow spot is exceedingly thin, so much so that 


24 


THE EYE ANATOMICALLY. 


the dark color of the choroid can be distinctly seen through it, 
and makes it almost present the appearance of an opening. 

About one-tenth of an inch to the inner side of the yellow 
spot is the point of entrance of the optic nerve, which is the 
only portion of the surface of the retina from which the power 
of vision is absent. It is important for the optician that he 
possess a clear idea of the relative positions of the yellow or 
sensitive spot and the optic nerve entrance or blind spot. 
Some persons think that the latter is directly in the center of 
the posterior part of the eye, but the fact is this point is occu¬ 
pied by the former (yellow) spot. These facts can perhaps be 
better impressed on the mind by the illustrations. 

The retina is a very delicate and extremely complicated 
structure. Though its greatest thickness does not exceed the 

of an inch, yet microscopists have described some eight or 
ten different layers. It will be sufficient for our purpose, how¬ 
ever, to consider it as composed of three layers: 

External; or, Columnar Layer. 

Middle; or, Granular Layer. 

Internal; or, Nervous Layer. 

The central artery of the retina, with its accompanying 
vein, pierces the optic nerve, and thus enters the cavity of the 
eye-ball. It immediately divides into four or five branches, 
and later forms a fine capillary network of blood-vessels. 


THE AQUEOUS HUMOR. 

The aqueous humor completely fills the anterior portion 
of the eye-ball, that is, the space included between the cornea 
in front and the crystalline lens and ciliary processes behind. 
It is small in quantity, and its composition is little more than 
water with a small quantity of chloride of sodium in solu¬ 
tion. The iris floats in the aqueous humor, that is, the humor 
is on all sides of it. 

If this fluid is evacuated by injury or operation, there is a 
fortunate provision of nature that it is reproduced very 
quickly, as otherwise the usefulness of the eye would be im- 


THE EYE ANATOMICALLY. 


25 


paired by the loss of convexity of the anterior portion of the 
eye, and by an adhesion of the iris to the cornea and lens. 

THE VITREOUS HUMOR. 

The vitreous humor occupies four-fifths of the interior of 
the eye-ball. It fills the concavity of the retina, and is hol¬ 
lowed in front for the reception of the crystalline lens. It is 
perfectly transparent and colorless, of about the degree of 
density of thin jelly, and consists of an albuminous fluid in¬ 
closed in a delicate transparent membrane, called the hyaline 
membrane. 

In health there are no blood-vessels in the substance of 
the vitreous body. On account of its consistency it is ad¬ 
mirably adapted to maintain the form of the eye-ball, and give 
to the retina, which is spread upon its outer surface, the neces¬ 
sary support, while at the same time it yields sufficiently to 
protect this delicate structure from injury by jarring or ex¬ 
ternal pressure. If “the eye runs out,” that is, if there is an 
escape of the vitreous fluid from rupture of the coat of the 
eye, it is not again re-formed (as is the aqueous humor). 

The maintenance of the proper form and distension of the 
globe of the eye depends largely upon the vitreous humor, 
without which the ball would collapse into a shapeless mass. 
It also keeps the choroid and retina in position, so that the 
latter shall be at the proper location to receive the images 
formed by the refracting media. 

The eye is a wonderful illustration of skilful packing, com¬ 
bining firmness, elasticity, compactness, mobility and safety, 
in a degree of perfection that can never be approached by art, 
and is perhaps scarcely equaled elsewhere in nature. 

THE CRYSTALLINE LENS. 

The crystalline lens, enclosed in its transparent and elastic 
capsule, is situated immediately behind the pupil, and in a 
depression in the front part of the vitreous humor, and is sur¬ 
rounded by the ciliary processes, which slightly overlap its 
margin. The lens in its capsule (hyaloid membrane) is sus¬ 
pended at all portions of its circumference, and is retained in 


26 


THE EYE ANATOMICALLY. 


its position chiefly by what is termed the suspensory ligament 
of the lens, which originates in the meshes of the ciliary body, 
and is firmly inserted at the edge of the capsule. 

The crystalline lens is a transparent, double convex lens, 
the convexity being greater on its posterior than upon its an¬ 
terior surface. It measures about a third of an inch trans¬ 
versely, and about one-seventh of an inch antero-posteriorly. 
It is firmer than the vitreous, but is not solid, the outer portion 
being softer, while that beneath is firmer, and the central por¬ 
tion forms a hardened nucleus. 

In young persons the consistency of the lens is such as 
to allow its shape or convexity to be readily altered, this con¬ 
traction and expansion being accomplished by means of the 
ciliary muscle. It grows denser with age, and hence is less 
susceptible to the action of this muscle; it also becomes flat¬ 
tened on its surfaces, slightly opaque and of an amber tint. 
These changes bring on the condition of presbyopia, as will be 
fully explained in Chapter X. 

The eye-ball is imbedded in a soft cushion of oily fat, 
which supports and protects it, and at the same time allows it 
to move in all directions as freely as if it floated in water. 

The orbit in which the eye-ball is lodged is a hollow cone 
of bone with its base directed forward and outward. The 
external edge of the orbit extends much less forward than the 
internal, and the axes of the two orbits, if followed back, would 
meet at an angle of 45 0 ; this arrangement permits the widest 
lateral range of vision consistent with the power of directing 
both eyes at the same time to a near object, that is, the faculty 
of binocular vision. The position of the orbit varies in dif¬ 
ferent animals, and in some the eyes are placed so entirely at 
the side of the head that they can see almost as well behind 
them as in front. They enjoy a wide range of sight, but do 
not possess the gift of binocular vision. 

The edges of the orbit are dense and strong, particularly 
the upper one, which overhangs the eye and is capable of 
shielding it from a powerful blow; as is illustrated in the case 
of a “black eye,” when the surrounding soft tissues are 
swollen, and inflamed and filled with blood, while the eye 
peeps through them quite unharmed. When the eye-ball 


THE EYE ANATOMICALLY. 


27 


itself is injured by the fist, it is always by a blow aimed from 
beneath. 

The appendages of the eye include the eye-brows, the eye¬ 
lids, the conjunctiva, and the lachrymal apparatus, consisting 
of the lachrymal gland, the lachrymal sac, and the nasal duct. 


THE EYE-BROWS. 

The eye-brows are formed of muscle and thick skin, cov¬ 
ered with coarse short hairs, and rest upon a bony ridge above 
the edge of the orbit. The hairs are arranged somewhat like 
the straw on a thatched roof, and serve to protect the eye from 
the perspiration that trickles down the forehead. The muscles 
of the eye-brows serve, to some extent, to control the amount 
of light admitted into the eye, as is evidenced by the way in 
which they are instantly drawn down when suddenly exposed 
to a dazzling light. 

They have aesthetic functions, too, as powerful organs of 
expression. A frown is produced by wrinkling and depress¬ 
ing the brows, while by elevating them incredulity, surprise 
or contempt can be expressed almost as plainly as by words. 
They may be considered almost distinctive in man, as they are 
not found in animals, and even our supposed forefathers, the 
monkeys, cannot lay claim to them. 


THE EYE-LIDS. 

The eye-lids are two thin movable folds, placed in front of 
the eye, to protect it from injury by their closure. The upper 
lid is much the larger and more movable of the two, while the 
lower one is almost stationary. The result from this difference 
in size is, that when the eye is closed the pupil is completely 
covered by the upper lid, and is, therefore, much better pro¬ 
tected than it would be if placed opposite the fissure between 
them. An additional protection is afforded during sleep, or 
when the eye is threatened with violence, by a rolling upward 
of the ball. 

The skin of the eye-lids contains no fat, which is a fortu¬ 
nate provision for stout people, for if fat increased there as it 


28 


THE EYE ANATOMICALLY. 


does sometimes in other parts of the body, the eye would be 
mechanically closed by the swelling and accumulation of adi¬ 
pose tissue. This mechanical blindness may result from the 
swelling caused by injury or disease, an example of which is 
seen in erysipelas of the face, and in rhus poisoning affecting 
this portion of the body. 

The eye-lids consist principally of plates of cartilage, 
called the tarsal cartilages, which afford the necessary support 
and maintain their shape. These are covered on the outside 
by a loose thin skin, and lined with a smooth and delicate 
mucous membrane, called the conjunctiva; in addition to 
which are the muscles that move the eye-lids. 

The lids lie gently upon the eye, and are maintained in 
accurate contact with it by atmospheric pressure. The two 
surfaces, that is, the outer surface of the eye and the inner 
surface of the lid, move upon each other with perfect freedom, 
and with an entire unconsciousness of friction in health, but 
which is quickly disturbed by the presence of the smallest par¬ 
ticle of dust. This absence of friction depends not only upon 
the smoothness of the surfaces and their exquisite adaptation 
to each other, but because they are lubricated by a secretion 
of mucus from the conjunctiva and moistened by a constant 
flow of tears. 

The polish and transparency of the cornea are preserved 
by frequent unconscious winking, which keeps its surface 
moist and free from dust. When the lids cannot be closed, 
because of paralysis of the muscle, the cornea soon becomes 
hazy and dim, from the evaporation of the fluid from its sur¬ 
face, and blindness may be the result. 

There are a number of small glands, about thirty in 
number, called the meibomian glands, situated upon the inner 
surface of the lids, and which open upon their free margin 
near the roots of the lashes, and thus serve to keep the edges 
of the lids greased with an oily secretion, which not only pre¬ 
vents their adhesion, but also impedes the overflow of tears. 
This latter effect may be seen by greasing the edges of a cup 
and then filling it with water, when it will be found that the 
surface of the water can be raised to a higher level than the 
edges of the cup, or, in other words, there will be no overflow 


THE EYE ANATOMICALLY. 


2 9 


in the greased cup with the same amount of water that quickly 
overflows a cup that has not been greased. 

EYE-LASHES. 

When the lids are partially closed the lashes come 
together in such a way as to form a kind of a screen, which, 
without excluding vision, serves as an admirable protection 
against wind and dust and excessive light. As their bulbs are 
freely supplied with nerves, they are delicately sensitive to the 
slightest touch, and act as “feelers” to warn the eye of the 
approach of any small object, as an insect in the dark, or when 
the vision is not on guard. 

Each eye-lash reaches its maturity in about five months, 
and then drops out and is succeeded by a new one. This 
process is greatly interfered with by inflammation of the edges 
of the lids, when the lashes come out more freely and are not 
renewed again, or if restored the new lashes take a wrong 
direction and turn their points against the surface of the eye¬ 
ball. This is the condition known as “wild hairs”; it causes 
much discomfort, and leads to serious damage to the sight by 
clouding the cornea. When the lashes are lost entiretv, the 
face has a peculiar and unnatural appearance. 

THE CONJUNCTIVA. 

The mucous membrane of the eye is called the conjunc¬ 
tiva; it is continuous with the skin at the margin of the lids; 
it lines their inner surface and then passes over to the ball, 
forming a loose fold. It covers the front part of the sclerotic, 
and lines the walls of the tear ducts, becoming continuous 
with the mucous membrane of the nose and throat, and hence 
they are all affected in “a cold in the head” or influenza. 

The conjunctiva is ordinarily nearly white, with perhaps 
a few ,of the larger blood-vessels seen winding through it, but 
it quickly becomes red and congested from local injury or 
inflammation, or by disturbances of the circulation of the head, 
as in the case of the blood-shot eye in the' morning that tells 
of the previous night’s debauch. 

The conjunctiva is the seat of many of the ordinary in¬ 
flammations of the eye, usually termed “sore eyes,” and of 


30 


THE EYE ANATOMICALLY. 


that intractable disease known as “granulated lids.” The 
characteristic yellow tinge of the eye in jaundice is due to the 
coloring matter of the bile being deposited in this membrane. 

The conjunctiva is the lodging place of foreign bodies 
that so frequently get into the eye and cause so much irrita¬ 
tion. Some of them become imbedded in its tissues, and have 
to be forcibly removed. Others drift along in the current of 
tears and lodge at the inner canthus, where they can be easily 
extracted. Sometimes it becomes necessary to evert the upper 
lid in order to discover the offending particle. 

The apparent size of the eye depends chiefly upon the 
width of the opening between the lids. The actual size of the 
ball varies but little in different individuals, but some eyes 
appear much larger than others because the lids are more 
widely separated. When an inflamed eye is kept constantly 
partially closed from an excessive sensitiveness to light, much 
anxiety is often caused by the fear that it is becoming smaller. 

A drooping eye-lid conveys the impression of weakness 
or fatigue, and in the final stages of wasting fevers the half 
closed eye, concealing the cornea and exposing only the white 
sclerotic, has a ghastly effect, and is considered a most dis¬ 
couraging symptom. 

THE LACHRYMAL APPARATUS. 

By the lachrymal apparatus is understood that series of 
structures which has to do with the manufacture of tears and 
the disposition of them by drainage into the nose. It consists 
of the lachrymal gland which secretes the tears, and the ducts 
which convey the fluid to the eye, which, after washing its 
surface, is collected and carried away by the lachrymal canals 
into the lachrymal sac, and thence along the nasal duct into 
the cavity of the nose. 

The lachrymal gland is located in a depression in the 
roof of the orbit at its upper and outer part; it is of oval form, 
about the size and shape of an almond. Its secretion is poured 
upon the ball through a number of small ducts, from seven to 
ten in number, which open by small orifices on the upper and 
outer half of the conjunctiva, the openings being arranged in a 
row, so as to disperse the secretion over the surface of the eye. 


THE EYE ANATOMICALLY. 


31 


.Under ordinary circumstances the tears are supplied in 
the usual quantities, but under the stimulus of emotion or a 
foreign body in the eye they are poured forth in large quanti¬ 
ties. The direction of the flow of tears is from above and out¬ 
ward, downward and inward. At the inner corner of the eye 
they drain into the lachrymal points (one on each lid), the 
canals from which converge into the lachrymal sac, which 
merges into the nasal duct, and passes into the nose. Ordi¬ 
narily we are not conscious of any moisture in the nose, but 
when there is a hypersecretion of tears we have a “running of 
the nose” and an overflow on the cheek. 

It is a curious fact that infants do not shed tears before the 
third or fourth month; and among the lower animals the ele¬ 
phant is the only one accused of this human weakness. 

MUSCLES OF THE EYE-BALL. 

The eye-ball is moved in various directions by the muscles 
of the eye, six in number, four straight and two oblique mus¬ 
cles, as follows: 

External Rectus Muscle. 

Internal Rectus Muscle. 

Superior Rectus Muscle. 

Inferior Rectus Muscle. 

Superior Oblique Muscle. 

Inferior Oblique Muscle. 

The superior rectus muscle is attached to the upper por¬ 
tion of the globe, and moves it in an upward direction; the 
inferior rectus to the lower portion, and moves it downward; 
the external rectus to the outer portion, and moves it outward; 
the internal rectus to the inner portion, and moves it inward. 
The superior and inferior oblique muscles are attached to the 
outer portion of the ball, and move it in a rotary or oblique 
manner. 

A shortening or contraction of one of these muscles 
causes the condition of strabismus or cross-eye. If the in¬ 
ternal rectus muscle is affected the eye turns in and converg¬ 
ent strabismus is the result. If the external rectus is affected, 
the eye turns out and divergent strabismus results. Some- 


32 


THE EYE ANATOMICALLY. 


times these conditions can be cured by glasses; at other times 
only by the division of the offending muscle by the surgeon’s 
knife. 

WHAT A WONDERFUL ORGAN. 

With the foregoing knowledge of the anatomy of the eye, 
the student can appreciate what a wonderful organ it is, and 
how admirably adapted for the purpose it is intended to serve. 
The sense of sight is the most remarkable of all our senses, 
both for the special nature of the impressions which it receives, 
the complicated structure of its apparatus, and the variety and 
value of the information which it affords with regard to 
external objects. 

It is by this sense that we receive the impressions of light 
and color, with all their modifications of intensity and combi¬ 
nation, and acquire our principal ideas of form, space and 
movement. The eye is also superior to the other organs of 
special sense in the rapidity of its action, and in the delicacy 
of the distinction which it is capable of making in the physical 
qualities of external objects; and it affords the most continu¬ 
ous and indispensable aid for all the ordinary occupations of 
life. 


CHAPTER III. 


OPTICS. 


Before taking up the consideration of the various defects 
of the eye and their correction by glasses, we must first give 
some attention to the study of the fundamental principles of 
the science of optics. 

Light is a force which emanates from luminous bodies 
and travels in straight lines in all directions, which lines are 
called rays. Every object we can see owes its visibility to 
rays of light proceeding from it to the retina. The bodies 
which emit rays of light, as the sun or a candle flame, are 
termed luminous. 

Those which only reflect rays, as the moon, a clock-face 
or a mirror, are illuminated. 

A ray is the smallest visible line of light. 

A beam is a collection or bundle of parallel rays. 

A pencil is a collection of converging or diverging rays. 

A single ray of light cannot be obtained, but a number 
together, that is, a beam or a pencil, may be easily demon¬ 
strated. Completely darken a room facing the sun, bore a 
small hole in the shutter, and a vivid, perfectly straight strip of 
light will be seen traversing the darkness, if the air of the 
room contain particles of solid matter like dust or tobacco 
smoke. 

The path pursued by the rays of light, whether from 
luminous or illuminated bodies, as long as they pass only 
through the atmosphere, is in straight lines and never in 
curves; but when they pass through other bodies or sub¬ 
stances they may be diverted from their original course, always 
retaining, however, their straightness. 

Luminous bodies are supposed to be composed of an in¬ 
finite number of luminous points giving off rays in all direc¬ 
tions. These rays are all divergent; in fact, strictly speaking, 

33 




34 


OPTICS. 


all light in nature exists in the form of divergent rays. There 
are in nature no converging rays, neither are there any abso¬ 
lutely parallel; although, as has already been stated, those 
rays that proceed from infinite distance are considered parallel, 
and, in fact, for our purpose as opticians, we consider those 
rays that proceed from a distance of twenty feet as practically 
parallel, or so little divergent that the difference can only be 
mathematically expressed, and it is impossible to show that 
they are not parallel. Therefore, in dealing with rays that 
enter the eye, it will be sufficiently accurate to consider them 
to be parallel if they proceed from a distance of twenty feet or 
more. 

It can be understood that the amount of divergence of the 
rays received on a given surface must be proportionate to the 
distance of that surface from the point whence the rays come. 



In this figure the two perpendicular lines represent two 
surfaces of equal dimension. A single look shows that the 
divergence of the rays proceeding from a luminous point L to 
the surface a b is greater than the divergence of the rays which 
are received on the surface c d. If this diagram be greatly 
enlarged, the divergence of the lines striking the surface c d 
will be so slight that, on isolating any small portion of them, 
they will appear almost parallel. Hence it can be easily under¬ 
stood that any two rays of light proceeding from a luminous 
point in the sun, distant more than ninety-four millions of 
miles, and entering the opening of the pupil of the human eye, 
which is less than a quarter of an inch in diameter, must form 
with one another so small an angle as not to be appreciable, 
and such rays (that is, rays proceeding from infinite distance) 
are, therefore, always assumed to be parallel. In optics, then, 
we have these two facts laid down as general principles: First, 
that all rays of light proceeding from a distant source are said 
to be parallel; and second, that all rays proceeding from a 
near source are divergent. 





OPTICS. 


35 


A ray of light moving through a homogeneous medium, 
such as the surrounding atmosphere, continues in a straight 
line forever, but if it meets with an opaque body, it may be 
■either absorbed or reflected, i. e., made to change its course at 
-an angle. If the opposition be so great that it cannot pass 
through the interposing object, it is reflected; while if the 
object be traversable it is refracted. 

Bodies are said to be transparent when they transmit light 
freely, so that objects can be distinctly seen through them, as 
.air, glass, crystals, etc. They are said to be opaque when no 
light is able to pass through them at all. Objects that break 
up the rays, but transmit a diffuse, softened light, as porcelain 
or ground glass, are called translucent. They do not allow the 
details of the image to be seen through them, but only the 
shade of its outlines. It is a curious fact that no substance is 
perfectly transparent; all absorb or quench at least a portion 
of the rays, while others are reflected or scattered. If an 
object should be perfectly transparent it would not be visible, 
because all the rays would pass directly through it, and there 
"would be no scattered rays to pass into the eye. Even the 
refractive media of the eye are not perfectly transparent. 
Neither are there objects perfectly opaque; the densest of 
metals, as gold, when beaten very thin, transmits a greenish 
light. 

Opaque bodies placed before a source of light cast 
shadows on the background. In consequence of the straight 
lines in which light always moves, the form of the shadow will 
correspond to the outlines of the object, but its size will vary 
according to the distance of the object from the source of light, 
and of the screen from the object. That the edges of a shadow 
may be sharply defined, the light must proceed from a point; 
if, on the contrary, it comes from a luminous surface, the 
borders of the perfect shadow are surrounded by an imperfect 
shadow. This can be readily shown by a lamp having a flat 
wick, so as to make a broad, thin flame. 

If the broad surface of the flame be turned toward a screen 
of white paper held two or three feet distant, and a small body 
like a knife-blade be interposed, the real shadow will be seen 
■surrounded by an imperfect one; while if the lamp be partly 


36 


OPTICS. 


turned so that the edge of the flame be toward the screen, and 
the knife-blade be again interposed, the shadow produced by 
it will be clear-cut and its edges perfectly defined. 

The intensity of illumination produced by rays of light 
diverging from a luminous point diminishes in proportion to 
the square of the distance. A screen one foot square, distant 
one foot from a lighted candle, receives a certain number of 
rays and is illuminated accordingly. A screen two feet square,, 
at two feet distance, receives the same number of rays, but they 
are spread over four times the surface; consequently each point 
on the second screen receives only one-fourth as many rays as 
a similar point on the first. So a screen three feet square, at a 
distance of three feet, receives the same number of rays as the 
smaller one at one foot, but they are spread out so as to cover 
nine square feet; hence the intensity of the illumination is but 
one-ninth of the first instance. This is well illustrated in the 
following figure: 



A represents the flame of a candle; i, 2 and 3 represent 
screens of one, two and three feet square, placed at correspond¬ 
ing distances from the flame. Each screen receives the same 














OPTICS. 


37 


number of rays; but while at I they cover only one square 
foot, at 2 they are spread over four square feet, and at j, nine 
square feet. 

It requires time for light to travel through space, although 
the rapidity with which it moves is so great that for distances 
which we are able to measure on the earth its passage seems 
almost instantaneous. According to the calculations of as¬ 
tronomers, light moves at the rate of about 192,000 miles in a 
second; hence it can be figured out that it requires a little more 
than eight minutes for the light of the sun to reach the earth. 

A ray of light meeting an opaque body may be either 
absorbed or reflected. If a piece of dark cloth be held at an 
angle of forty-five degrees to the beam of light admitted by a 
minute hole in the shutter into a darkened room, the light will 
be absorbed or disappear in the cloth. But if a mirror be 
placed in the same position, the light will be reflected and the 
beam will pass onward at right angles to its original course. 
Reflection of light varies in degree according to the quality 
of the surface upon which the light falls; rough, dark-colored 
surfaces reflect light very imperfectly, while light-colored and 
polished surfaces reflect very perfectly. 

When light falls perpendicularly on any plane polished 
surface, as a mirror, it is thrown back and exactly retraces its 
first course. If a ray strikes the surface obliquely, as in the 
above instance, it is reflected obliquely. If, now, a ruler be 
placed perpendicular to the surface of the mirror at the point 
where the beam impinges, it will divide the angle made by 
the beam into two equal parts, as shown in the figure on 
page 38. 

If the mirror be more inclined, with the ruler still remain¬ 
ing perpendicular to its surface, the angles will be larger, but 
still equal. If the inclination of the mirror be diminished, the 
angles will be smaller, but still equal; SO' that we find that the 
angle on one side of the ruler is always equal to the angle on 
the other side; or, in other words, the impinging ray and the 
reflected ray each forms a similar angle with a line perpen¬ 
dicular to the surface at the point of incidence; hence, the law 
which applies to the reflection of light is expressed by saying 
that “the angle of reflection is equal to the angle of incidence.” 


38 


OPTICS. 



In regard to the variation with which different bright- 
polished bodies reflect light, while black and dark-colored 
bodies absorb it, it may be said that if all the light would be 
absorbed by any object the object would be invisible, because 
there would be no rays left to enter the eye. 

In the case of the mirror just mentioned, it is a flat surface 
reflecting light; but no surface is absolutely flat or plane; the 
denser metals can be made to approach the nearest to this con¬ 
dition, for which reason reflection from their smoothest sur¬ 
faces is nearly perfect. When the smooth surfaces of objects, 
like polished wood or marble, are examined with the micro¬ 
scope, they are found to consist of an infinite number of small 
planes, inclined to each other at all possible angles. These 
planes scatter the light in every direction, and produce what is 
called diffuse light. 

If a sunbeam pass through a perforation in the shutter of 
a dark room and impinge upon a highly polished metal surface, 
it is almost entirely reflected and strikes the wall, making a 
bright spot the same size as the beam, while the room remains 
dark. If a sheet of white paper be substituted for the polished 
surface, there will be no reflection of the beam as before, but 








OPTICS. 


39 


instead there will be a faint general illumination all over the 
room. It is these irregularly-reflected or scattered rays that 
make non-luminous objects visible when they are illuminated. 

In a plane mirror the reflected image appears as far 
behind its surface as the object is in front of it. The image is a 
perfect representation of the object in form and size and color, 
but it is laterally transposed, so that the left of the object be¬ 
comes the right of the image, and the right of the object 
appears as the left of the image. When we stand before a 
mirror, the left half of the face appears as the right in the 
image, and the right half as the left in the image. The 
reflected image of a printed page shows the letters arranged 
backward, and from right to left; they appear just as the com¬ 
positor arranges his type. If this image be received and re¬ 
flected by another mirror, the letters of the page are again seen 
in their accustomed position for reading. When the type is 
arranged for the printing press, a mirror enables us to read 
them the same as if their impressions were on paper. 

REFLECTION FROM CURVED SURFACES. 

The law that the angle of reflection equals the angle of 
incidence holds good when applied to regularly curved sur¬ 
faces, for a curved surface may be regarded as a number of 
infinitely small planes inclined one to another. Each plane, or 
rather each point of the curved surface, would reflect a ray of 
light according to the law that has been stated. If the mirror 
has a concave parabolic surface, rays parallel to its axis will 
converge after reflection and cross each other on the axis at a 
certain distance from the reflector, and then proceed in a 
divergent direction. The point of crossing is called the focus, 
and its distance from the mirror, the focal distance. 

The mode in which a concave surface reflects can be 
easily understood by enlarging a few of its planes, as shown in 
the figure on page 40. 

If we consider the angle of incidence for each, it will be 
seen that parallel rays falling upon three planes inclined to one 
another, as a, b and c, would be reflected to a point situated as 
at f, and what is true of the three planes is approximately true 


40 


OPTICS. 



of the infinity of plane surfaces contained in the curved surface 
at D. Parallel rays falling upon a concave mirror are reflected 
to a point situated upon the axis. 

If the concavity is the segment of a sphere, rays issuing 
from a luminous point situated at its center would be reflected 
back upon themselves, as the radii are obviously perpendicular 
to all parts of the surface. But if the impinging rays are par¬ 
allel, they would be reflected to a point situated at about one- 
half the length of the radius from the mirror surface, as seen 
in the figure, and, vice versa, divergent rays from a point at f 
would be reflected as parallel. This point is termed the prin¬ 
cipal focus of the mirror. 

If the rays that strike a concave surface are already diverg¬ 
ent, they would be reflected to a point beyond the principal 
focus of the mirror. It is obvious, on a little thought, that the 
closer any luminous point approaches the mirror (and conse¬ 
quently the more diverging the rays that proceed from it) the 
farther its focal point will recede. And if the luminous point 
were to be situated at this focal point, the rays would be 
reflected back to the position of the first luminous point. 
These two points are called conjugate foci, and have a constant 
relationship, so that if the principal focal length is known, and 
the distance of one focus is given, the other can be determined 
by a mathematical formula. 

With convex mirrors all the above facts are reversed. In 
following the rays we have hitherto seen them converge to a 
real or positive focus; but in the case of a convex spherical 
surface parallel rays are rendered divergent, and, therefore, 
no positive or real focus is formed. But if the divergent rays 
are continued by imaginary lines to the far side of the mirror, 
they will be found to meet in a point corresponding to the 












OPTICS. 


41 


principal focus of a concave mirror of the same curvature. As 
the focus is imaginary and on the opposite side of the mirror 
from the actual rays, it is termed negative; and as the diverg¬ 
ent rays have the same optical properties as if they came from 
the negative point, the focus is termed virtual. 

The image is formed in the focus of the rays proceeding 
from the object after being reflected; in the plane mirror the 
image is virtual and as far behind as the object is in front of 
it; in the concave mirror the image is on the same side as the 
•object. 

REFRACTION OF LIGHT. 

When a ray of light passes from one homogeneous me¬ 
dium to another homogeneous medium of different densitv 
from that of the first, it is bent at the surface of separation, and 
proceeds in the second medium in a straight line, but in a 
•changed direction. This is true of all rays that do not fall 
perpendicularly to the surface of separation. Those rays 
"which are perpendicular to the surface are not bent, but pro¬ 
ceed through the second medium in an unchanged direction. 
Rays which fall obliquely on the surface form an angle with the 
-perpendicular at the point of incidence, called the angle of inci¬ 
dence. At the surface of the second medium the rays take 
•another direction and form a different angle with the perpen¬ 
dicular, and this is called the angle of refraction, and it usually 
is not equal to the angle of incidence. These two angles, that 
is, the angle of incidence and the angle of refraction, always 
have definite and fixed relations to each other, and it is the 
enunciation of these relations that constitutes the laws of 
refraction. 

When a ray of light passes obliquely from a rarer to a 
denser medium, it is generally bent toward the perpendicular; 
on the contrary, when it passes from a denser to a rarer me¬ 
dium, it is bent from the perpendicular. This is shown in the 
figure at the top of page 42. 

G H represents a strip of plate-glass with parallel surfaces, 
and 0 P the perpendicular. A ray of light falling in the direc¬ 
tion 0 P passes through the glass unrefracted. B I is an inci¬ 
dent ray passing through the air (which is a very rare medium) 
;and falling on the surface of the glass (which is a very dense 


42 


OPTICS. 



medium) at the point /, where it is bent toward the perpen¬ 
dicular and proceeds in a straight line to E. Here it again 
meets the air (the first medium), and is bent from the perpen¬ 
dicular and continues in a line parallel to its first course. 

The more obliquely the light falls on the refracting sur¬ 
face, the greater is the amount of refraction which its rays- 
undergo; hence, the degree of refraction varies with the angle 
of incidence, but the relation that one bears to the other re¬ 
mains unchanged. All the rays of light impinging on a re¬ 
fracting surface do not enter it; a part are reflected or thrown 
back into the first medium. The larger the angle of incidence, 
the greater will be the number of reflected rays. 

If the surfaces of a refracting substance are inclined to one 
another, as in a prism, it is obvious that a ray can never be 
perpendicular to both surfaces at the same time, and, hence, a. 
ray can never fall upon a prism in any such way as not to be 
refracted. 



In the figure we see a ray of light falling from the point a 
perpendicularly, to the first surface of the prism, and it, there¬ 
fore, undergoes no refraction, and if the second surface was 










OPTICS. 


43- 


parallel to the first, the ray would undergo no deviation. But 
011 emerging from the second surface it comes into relation 
with the second perpendicular, from which it is refracted ac¬ 
cording to the law of passage from a denser to a rarer medium. 
A ray from a would, therefore, not proceed to b, but to c, and 
would vrtually come from a point situated as at d. Rays of a 
moderate degree of obliquity are refracted toward the base of a 
prism, and this fact will enable us to understand the laws of 
refraction by curved surfaces, on which the main properties of 
the eye, as an optical instrument, depend. 

If two prisms are placed base to base, as shown in the 
following figure, any two parallel rays of light, as a and b y , 
falling upon corresponding points in the surfaces, being 
equally refracted toward the base, would meet and cross at 
some point situated as at c on the far side. 



This point may be termed the focus for the two rays; and’ 
as the reverse of every optical fact holds good, so any two^ 
diverging rays proceeding from a luminous point, as at r, 
would, in passing through these prisms, be rendered parallel,, 
as a and b. 

If the surfaces of the prisms, instead of being plane, were 
curved equally from the center to the edge, there would be 
formed an optical contrivance called a lens, which, in the above 
case, where the bases of the prisms are joined together, would 
be a bi-convex lens. As a curve can be resolved into a 
number of small planes, it follows that a bi-convex lens may be 
regarded as a number of truncated prisms arranged with their 
bases toward the center. Prisms so arranged would refract 
parallel rays toward their bases; hence, parallel rays falling on- 
one surface would be rendered convergent and tend to meet in* 






44 


OPTICS. 


a point at some distance from the second surface—this is called 
the principal focus, and its distance from the optical center of 
the lens its principal focal distance. Lenses are often spoken 
of according to their focal length, as two, four, or eight-inch 
lenses. 

Now, if the surfaces of a lens are perfectly round, being 
sections of a perfect sphere, it is called a spherical bi-convex 
lens, and the ray of light that passes directly through the 
center, in line with the perpendicular, is not refracted, and is 
called the axial ray of the lens, or the principal axis. In a 
bi-convex lens the parallel rays from infinity, coming from a 
given direction, will fall upon the convex surface of the lens, 
and, passing through, will be bent by the lens until all the rays 
will meet at the principal focal point, and then, passing on, will 
diverge; while the axial ray, which passes through the center 
and strikes the surface of the lens parallel to its perpendicular 
at that point, will pass onward without any deviation. We 
may also have certain rays of light striking the lens on other 
portions of its surface, and which pass through without refrac¬ 
tion, as all those rays that enter the lens parallel to the per¬ 
pendicular at the point of entry are not refracted. These rays 
are called the secondary axes, and all rays passing parallel to 
them will be brought to a focal point on each secondary axis. 

All these secondary axes must pass through the center of 
the lens at this nodal point,* and we find that all rays of light 
that coincide with the principal or secondary axes are not 
refracted, but all other rays which are parallel to these axes, 
when passing through a spherical bi-convex lens, are brought 
to a focus at the center of curvature of its curved surfaces. 
These focal points always rest on the principal or secondary 
axes, the first being the principal focal point, and its distance 
from the optical center of the lens represents the principal 
focal distance, or, in other words, the refractive power of the 
lens. 


* Nodal Point.—A nodal point is a point of intersection of convergent 
rays of light, and the axial ray passing through such a point is not refracted. 
The nodal point of the eye is situated just in front of the posterior surface 
of the crystalline lens, where all rays of light entering the eye intersect. 


OPTICS. 


45 


When we have reference to a lens of any kind, and speak 
of the focal distance, you will understand that a twelve-inch 
lens, for instance, has its focal point at twelve inches from the 
optical center of the lens. 

Now, every lens, either positive or negative, has prac¬ 
tically two nodal points situated on the axis, and called the 
anterior and posterior. These two points coincide with the 
two principal points situated on the principal axis at the optical 
center of the convex surfaces. Hence, all the rays of light that 
strike the surfaces of the lens, directed toward a nodal point,, 
will pass through the optical center of the lens and emerge as. 
if they came from the other nodal point, in a direction parallel 
to that of the incident ray. 

Divergent rays from a luminous point situated at the prin¬ 
cipal focus of a lens are rendered parallel on passing through 
the lens; but rays from a more distant point are less divergent,, 
and the refractive power of the lens is then more than suffi¬ 
cient to render them parallel. They are, therefore, rendered' 
convergent to a point situated at a certain distance on the 
other side of the lens; and as the relationship of these points is 
constant and interchangeable, they are termed conjugate — 
both are positive and real. 

On the other hand, the rays from a luminous point closer 
than the principal focus are too divergent to be rendered par¬ 
allel by the refractive power of the lens, and, consequently,. 
they emerge from the other side divergent, though, of course,, 
in a less degree than before passing through the lens. 



In this figure we see rays diverging from a luminous point 
B, which is situated on the axis of the lens and nearer to it 
than its principal focus c. If the resulting divergent rays a a 



46 


OPTICS. 


are continued backward by imaginary lines, they will meet in 
a point situated as at D, and the lines a a will have the optical 
value as if they proceeded from D, and not, as they really do, 
from B. The closer B is brought to c, the farther D recedes, 
and vice versa. A luminous point situated as at B has, there¬ 
fore, a conjugate focus on the same side of the lens. It has no 
real existence, but represents the point whence the rays seem 
to come—it is negative or virtual. 

A concave lens is thicker at the edge than at the center, 
and may be regarded as representing a number of prisms with 
their apices together at the center and their bases outward. 
Now if we apply to these curved surfaces the same rules of 
refraction as in the case of the convex lenses, we will find that 
as the parallel rays emerge from such a lens they are divergent, 
because they are bent toward the bases of the prisms. If the 
lens surface be spherical it forms a bi-concave spherical lens. 
This lens will so refract rays of light that they will diverge in 
all directions, as if they came from some point behind the lens; 
and it will be found that if the directions of the divergent rays 
are produced backward, they will meet in a point on the axis 
called the principal focus. But such a lens has no real or 
positive focus, and it is consequently called a negative lens. 

The distance of this backward focal point from the optical 
center of the lens is called the negative focal distance, which 
may be represented by inches or dioptrics. If we represent 
this distance in inches, then a bi-concave spherical lens of 
twelve inches focal distance will cause parallel rays of light to 
diverge, after they have passed through the lens and been 
refracted, as if they came from a point twelve inches behind the 
lens, on the principal axis. 












OPTICS. 


47 


The figure on page 46 represents a bi-concave lens C, with 
parallel rays from A, which pass through the lens and, being 
Tent toward the bases of the prisms (of which the concave lens 
is composed), are made divergent, as at B, with a direction as 
if they came from the negative focal point D , as shown by the 
■dotted lines. Thus, when the parallel rays strike these curved 
surfaces, they are bent in the same manner as when they strike 
the surface of the bi-convex lens, but the curvature here is 
different, as the bases of the prisms are now outward; there¬ 
fore the direction' of the rays is divergent as they pass through 
the lens. 

All lenses refract light on the same principle and in the 
same manner, according to the curved surfaces that are pre¬ 
sented to the rays of light; those that are convex bringing the 
rays to a postive focus, and those that are concave causing the 
rays of light to diverge as if they came from the negative focal 
jpoint, that is, behind the lens. 


Let us look for a moment at what are called Meniscus 
lenses. The first one is concavo-convex, and has a negative 
and a positive curved surface; but the curvature of the posi¬ 
tive surface being so much greater than that of the negative 
surface, the rays of light, after they pass through the lens, are 
brought to a positive focal point; while with the other lens, 
which is called convexo-concave, the negative surface has the 
greater refractive power, and hence the rays, as they pass 
through the lens, diverge from the negative focal point. 

By way of illustration, we will suppose that the concavo- 
convex lens above referred to has a curvature on its negative 
or concave side equal to a concave lens of two dioptrics, and 
that the curvature of the positive or convex side of the lens is 
■ equal to a convex lens of four dioptrics; thus the positive 
focal power of this Meniscus lens will be equal to the difference 


48 


OPTICS. 


between the two lenses; or, in other words, the strength of the* 
convex surface will be diminished or in part neutralized by 
the strength of the concave surface; or the two dioptrics, taken, 
from four dioptrics, leave two dioptrics as the strength of this 
Meniscus lens. As is well known, most of the lenses of the 
spectacles and eye-glasses of the shops, particularly lenses of 
low power, are ground according to this method. 

The particular advantages these Meniscus (or periscopic)' 
lenses are said to possess, are that they give much more correct 
secondary axes, and when adjusted to the eye yield more per¬ 
fect vision through the periphery of the lens, rendering the- 
field of vision much larger and more distinct. 

It should be known that the angle of refraction is always 
the same when passing through a concave or convex lens. It 
will be remembered that when the luminous point is at the 
focal distance of a convex lens, the rays, as they emerge from 
the lens, are parallel; but if the luminous point is moved far¬ 
ther back from the lens, then we find that the rays that emerge 
are convergent; while if the luminous point is moved nearer 
to the lens than the focal distance, the rays as they emerge are 
divergent. 



These facts are well shown in the above figure, where A 
represents a bi-convex lens, whose focal point is situated at B v 
the rays from which point will pass beyond the lens in parallel 
lines c c. Now, if the luminous point is moved back to D, it 
will be found that the emergent rays are convergent, the angle 
of refraction being the same, and if continued would meet at 
E. Then, again, if the luminous point be moved nearer to the 
lens and inside the focal point, as at F, the emergent ray* 







OPTICS. 


49 


would pass beyond the lens in a divergent direction, as the 
lines g g } the angle of refraction remaining the same as when 
the rays proceeded from the principal focal point. 

The result with a concave lens is the same; but when the 
luminous body is nearer the lens than its focal point, the 
emergent rays are more divergent than the refractive power of 
the lens, and they can not be made convergent. 

This fact of the angle of refraction being always the same 
is beautifully illustrated in the human eye, where the refraction 
of the dioptric media, taken collectively, represents a bi-con- 
vex lens of the same focal power. 

With a clear understanding of the optical principles which 
have so far been stated, the student is now in a position to 
understand the formation of images. When you look into a 
looking-glass you can see yourself—you can judge of your 
complexion, of the condition of your hair and clothes, or you 
can assume one or another peculiar attitude, with the certain 
assurance that whatever you see in the glass is an exact optical 
reproduction of yourself. But you must be aware that the 
image you see in the glass has no material existence, can 
neither touch nor be touched, is neither hot nor cold, solid nor 
gaseous, nor can it convey any impression to any other sense 
than sight—it is simply your image. 

If a sheet of white paper is placed six inches distant from 
a candle flame, and a cardboard screen exactly midway be¬ 
tween the two, with a pin-hole in its center, the image of the 
flame will be seen on the paper. Its color, shape and move¬ 
ments will be accurately reproduced, but it will be inverted. 

In the figure on page 50 we see numerous divergent rays 
proceeding from the luminous points in the candle flame A. 
Most of them are intercepted by the screen B, and it is evident 
that not more than a few rays from each point can pass 
through the pin-hole at c, and those that pass must necessarily 
cross there. The ray from the upper point of the flame A 
becomes the lower point on the paper P, and vice versa; and 
as the rays from the right cross over to the left, and those from 
the left cross over to the right, it follows that the image will be 
completely reversed. The illustration shows that if the screes 
is exactly midway between the candle and the paper. th< 


5o 


OPTICS. 


image and the flame will be exactly the same size; but if the 
paper is placed at a greater distance, the image will be en¬ 
larged; while if it is placed nearer, the image will be dimin¬ 
ished. A fact to be noted is that the smaller the image the 



If a second pin-hole is made in the screen, the result will 
be that two images will be formed on the paper; if a third pin¬ 
hole is made, three images will result, and so on. If the pin¬ 
holes are very close together, a corresponding number of 
images will be formed, but they will overlap and present only 
the appearance of a blurred spot of light. It can, therefore, be 
understood that the reflection from a sheet of paper, or from 
any visible object, is composed of the rays proceeding from a 
number of overlapping images of the sun, candle-flame, or any 
other source of illumination. With a minute aperture in a 
screen all superfluous rays are cut off, and each point of the 
flame is represented by a point in the image, which is, there¬ 
fore, clearly defined. 

It will be noticed that whereas the image of the flame 
appears on the screen and inverted, one’s own image appears 
to be behind the mirror and erect. 

In the figure on page 51 we see diverging rays proceeding 
from a candle-flame A, and falling in an oblique direction 
upon a plane mirror. As the angle of reflection is equal to the 
angle of incidence, the rays, as they fall upon the mirror and 
are reflected, will continue to diverge at the same rate as they 
did before reflection. But if the lines are produced on the 







OPTICS. 


51 


opposite side of the mirror, they will be found to meet at a 
point d, exactly the same distance behind the mirror as A is 



in front of it. The same is true of rays proceeding from other 
points. Hence the reflected rays seem to come from the 
points behind the mirror, and not, as they really do, from those 
in front. Such an image is termed virtual, in contradistinction 
io one that can be thrown on a screen and is termed real. 




CHAPTER IV. 


LENSES. 


The lenses that are used in spectacle frames are either 
periscopic or double. Periscopic lenses are concave on the 
inside and convex on the outside, the concavo-convex having 
a shorter radius of the convex surface than the convexo-con¬ 
cave. Double lenses are alike (that is, of similar curvature) on 
both sides—the convex lenses being convex on both sides, and 
the concave lenses concave on both sides. These double 
lenses are called also bi-convex and bi-concave. 

The ideal way to correct an optical defect would be to 
place the correcting lens into the eye and to make it an integral 
part of the eye. This being impossible, it is placed in front of 
the eye, as the only thing that can be done; but, unfortunately, 
it cannot move with the eye, and hence, to get the full benefit 
of the lens and to avoid the disturbing effect of looking 
through the edges of the glass in looking around, the head 
must be turned rather than the eye. With periscopic lenses 
there is, perhaps, a freer range of the eye behind the glass, and 
hence periscopic lenses are considered preferable to double, 
except in the strong numbers; but not universally, however. 
There seems to be a growing sentiment in favor of double 
lenses, and opticians are beginning latterly to look with more 
favor on them for general use. They sometimes order one 
and sometimes the other, according to special cases. 

There is an interesting fact in connection with periscopic 
and double lenses that should be known to every optician, and 
that is that the eye may become so much accustomed to wear¬ 
ing one kind as to be unable to wear the other. Many cases 
like the following are known: A person is wearing glasses 
that seem in every way suitable; he goes to the optician to 
buy a new pair exactly like the old ones, either because the old 

52 




LENSES. 


53 


ones are worn out, or because he wants to have an additional 
pair. He at once finds that the new ones are uncomfortable, 
and is soon unable to wear them at all. He goes back to the 
optician and tells him the glasses are not right and he cannot 
wear them. The optician measures the focus and finds it the 
same as the old glasses, and assures his customer that the new 
glasses are all right, are just like the old ones, and advises a 
perseverance in their use. The customer tries the new glasses 
again, with the same unpleasant result, until he becomes dis¬ 
gusted with himself and full of blame for the optician. Now 
the secret of it all is that the customer had, perhaps, been 
accustomed to wearing periscopic lenses, while the new ones 
given him were double; or, perhaps, the old ones were double 
and the new ones periscopic. 


MATERIAL FROM WHICH LENSES ARE MADE. 

In regard to the question of the material for the com¬ 
position of lenses, and in order to understand the statements of 
some opticians who advertise they manufacture certain kinds 
of lenses, it is well to know that all lenses are made from only 
two materials—glass and rock crystal or pebble. 

The great desideratum is to obtain a material that dis¬ 
perses light the least in proportion to its refractive power. 
'Crown glass answers this purpose best, and hence should be 
used for all the stronger numbers. The claim for the prefer¬ 
ence for pebbles rests solely on the ground that because they 
are harder their surfaces polish better, and hence do not 
scratch so easily; but the claim that they are better for the eye 
rests on no good foundation. Oftentimes the so-called pebble 
spectacles that have been bought at a high price from some 
itinerant optician, and that afford so much comfort to the 
wearer’s eyes, are found, when tested, to be nothing more than 
glass; so that we are compelled to the conclusion that (some¬ 
times, at least) the superiority of the pebbles is in the imagina¬ 
tion and not in the spectacles. 

Pebble-testers are made, to determine the composition of 
lenses. A common, every-day test is that pebble seems colder 
to the tip of the tongue than glass. 


54 


LENSES. 


VARIETIES OF LENSES. 

Lenses are of three kinds—spherical, cylindrical and pris¬ 
matic. The spherical and cylindrical are either convex or con¬ 
cave, while the prismatic may be plain or may be ground 
convex or concave; and any two or all three of them may be 
combined together into one lens. 



A, plano-convex lens; B, bi-convex or double convex 
lens; C, periscopic convex or concavo-convex lens; D, plano¬ 
concave lens; E, bi-concave or double concave lens; F, peri¬ 
scopic concave or convexo-concave lens. 

In writing about lenses, in order to avoid the constant 
repetition of the words convex and concave, it is customary to 
distinguish lenses of the former kind by the prefix of the plus 
sign (+), and those of the latter kind by the prefix of the 
minus sign (—). + 3 D. signifies a convex lens of three 

dioptrics, while — 3D. signifies a concave lens of three 
dioptrics. 

The letter S, or abbreviation Sph., is used to denote a 
spherical lens; and the letter C, or abbreviation Cyl., a cylin¬ 
drical lens. If neither is mentioned, it is understood to be 
spherical. 

If a revolving sphere be made to act as a grinding instru¬ 
ment, and a piece of plain glass be held against it and ground 
on its one surface, a plano-concave lens will result. If both 
sides are ground, a bi-concave lens will be produced. 

If the inner side of a piece of plane glass be ground on the 
outside of a small sphere, and the outer side of the same piece 





LENSES. 


55 


of glass be ground on the internal surface of a much larger 
sphere, a penscopic concave lens is produced. 

If a piece of plane glass be ground on both sides against 
the inner surface of a hollow revolving sphere, a bi-convex lens 
is produced. 

If a piece of plane glass be placed against a revolving 
cylinder, and ground until it fits the cylinder, a plano-concave 
cylindrical lens will be produced. 

If the outer side of a piece of plane glass be ground on 
the inner surface of a hollow revolving sphere, and the inner 
surface of the same glass be ground on the outer surface of a 
much larger sphere, a periscopic convex lens is the result. 

If a piece of plane glass be placed against the inner sur¬ 
face of a section of a hollow revolving cylinder, a plano-convex 
cylindrical lens will be produced. 

Cylindrical lenses act only upon the rays of light that fall 
in the meridian at right angles to the axis upon which the glass 
is ground. 

Glasses are frequently ground spherical on one side and 
cylindrical on the other, such glasses being called compound 
cylindrical lenses; or they may be ground convex-cylindrical 
with the axis in a certain direction on one side, while the other 
side may be ground concave-cylindrical with the axis in a 
different direction, such lenses being known as cross-cylindrical 
lenses. 

It should be kept in mind that one lens of a given focus is 
precisely like another of the same focus, and, consequently, 
glasses that are advertised under various high-flown names, as 
possessing special characeristics, have really no such charac¬ 
teristics at all that are not common to all glasses, and the con¬ 
clusion is forced that such advertisers are most likely frauds 
and humbugs. 

The chief use of lenses is to produce images. Hold a con¬ 
vex lens at a greater distance from a printed page than its focal 
length; for instance, a two-inch lens at six inches. The letters 
will appear inverted and diminished, and as if printed on the 
surface of the lens. Move the lens a little closer, say to four 
inches; the print will still be inverted, but the letters are no 


56 


LENSES. 


longer diminished, but of their natural size. At three inches 
the letters are magnified, and continue to increase as the prin¬ 
cipal focus is approached, that is, at two inches, where the 
inverted image disappears. If the lens be held closer than its 
focal length, the print will appear enlarged and erect. 

The lenses heretofore spoken of have been spherical, 
equally curved in all directions, with their refractive power 
exactly the same in all meridians, so that the rays are either 
brought to a focus or diverged as from a negative focus. 

But for the purpose of correcting different errors of re¬ 
fraction in the eye, there are found in all complete cases of 
trial glasses, sets of lenses whose action is quite different from 
that of those hitherto described. These are called cylindrical 
lenses, as they are practically segments of a cylinder with the 
axis of the cylinder at right angles to the refracting surface; 
they are generally plane on one side, with the refracting sur¬ 
face on the other, and may be either convex or concave. 

In studying the action of cylindrical lenses, we must con¬ 
sider chiefly all the rays as passing in two principal planes at 
right angles to each other. While the light also passes in any 
number of intermediate planes, yet the rays are so bent that 
in the convex cylindrical lens they will focus at a positive 
point, there forming simply a line, and not a single point, as in 
the case of a spherical lens. 

The two principal planes of the eye are generally vertical 
and horizontal, and it is to be remembered that the principal 
planes are always at right angles to each other, and may be at 
any degree of the arc of a circle. If we have a glass whose 
refractive power will be only on the rays of light of one 
meridian, the rays that pass in the meridian at right angles to 
that will pass parallel and unrefracted. 

As has just been stated, a cylindrical lens is one that is a 
section of a cylinder. If we take a cylinder of glass, with the 
axis running directly through its center, and cut off a section 
parallel to this axis, the rays of light that pass through in a 
plane that is the same as the axis will not be refracted; but 
all those passing at right angles to that plane will be either 
convergent or divergent, according to the refracting power of 
the glass and the radius of its curvature. 


LENSES. 


57 


B 



In the above diagram can be .seen the end of the cylinder 
of glass with its axis at C. If a section is made at B, the part 
cut off will form a plano-convex-cylindrical lens. A section 
made through this at A will present the surface of a rectangle, 
and all the rays in that plane will strike the glass parallel to its 
perpendicular, and will not be refracted. 



The action of such a lens is illustrated in the above dia¬ 
gram, which shows how parallel rays of light are refracted 
when passing in a plane at right angles to the axis .of the glass. 




--—— =? 









While at B the parallel rays of light are unrefracted when 
passing in a plane coincident with the axis. 












































58 


LENSES. 


Hence we have this rule, that a cylindrical lens will con¬ 
verge or diverge only those rays of light that pass at right 
angles to its axis, according to the refractive power of the lens; 
consequently the refracted rays of a cylindrical lens are never 
brought to a focal point, but form a straight line on a screen 
placed at its focal distance. 

THE ACTION OF PRISMS. 

The educated optician should thoroughly understand the 
action of prisms singly and combined with convex or concave 
glasses. As has already been shown, a prism deflects light 
toward its thick edge, or, in other words, an object seen 
through it has its position apparently moved toward the side 
of the thin edge. 

If an object is placed directly in the middle line in front of 
a person, so that a straight line drawn from it would strike the 
root of the nose, and if then one eye be closed and a prism 
held before the open eye, the position of the object will be 
apparently altered. If the prism be placed in front of the eye, 
base in, the object will be moved outward; while, on the other 
hand, if the prism be placed over the eye, base out, the object 
will be moved inward. The amount of displacement will, of 
course, depend upon the distance of the object from the eye 
and the degree of the prism. A prism of the proper degree 
with its base in will change the apparent position of the object 
from the middle line directly in front of the nose outward to a 
point directly in front of the eye, provided always that the eye 
is directed straight forward and its convergence is at rest. 

A similar prism placed in a like position before the other 
eye will have a similar effect, thus resulting in.placing the two 
objects directly in front of each eye, and single vision of the 
object will be afforded without the slightest effort of converg¬ 
ence. The object, however, being somewhat close to the eyes 
calls for an effort of the accommodation in order to be clearlv 
seen, and then we have a condition in which there is tension 
of the accommodation with relaxation of the convergence, a 
condition which could not be long maintained without fatigue, 
on account of the well-known correlation existing between the 
functions of accommodation and convergence. If, now, in this 


LENSES. 


59 


condition, a pair of convex lenses, of a focal length of the same 
number of inches as the object is placed from the eyes, be 
added to the prisms, they will remove the necessity for any 
effort of the accommodation, and, as the prisms had removed 
the necessity for convergence, the object is, consequently, seen 
for an indefinite period without any muscular effort on the 
part either of the accommodation or convergence. When a 
convex focus is ground upon a prism as above described, the 
combination is called “orthoscopic” spectacles. 

It will be manifest on a little reflection that every lens 
must have a corresponding prism, which would stand in “or¬ 
thoscopic” relation to it. In the stronger numbers of lenses 
the degree of the corresponding prism would be so great, and 
would add so much to the weight and thickness of the glass, as 
to practically prohibit their use, and limit the combination to 
the weaker numbers. Practically these glasses are not used 
very much, perhaps not as much as they deserve to be. 

The following table, which is only approximate, gives the 
number of the convex glass, with the degree of the corre¬ 
sponding prism: 


Lens. 
.50 D. 
1. D. 

1.25 D. 
1.75 D. 

2.25 D. 
3. D 


Prism. 

• itf° 

• 3 0 

• 4 / 4 ° 

. 6 0 

• 

• 9 0 


The test of such glasses being perfectly “orthoscopic” is 
that the two lenses, when fixed in their frame, should cast only 
a single image upon a card placed at their focal length; it is 
at once evident that this requires careful adjustment. 

The reason why only one image is formed by orthoscopic 
lenses is because these prismatic convex lenses are, or may be 
considered to be, eccentric portions of one very great lens, as 
shown in the accompanying diagram: 












6o 


LENSES. 


These portions representing the orthoscopic lenses are the 
parts through which the eyes would look if one large lens 
was held up before the face, with its center opposite the root 
of the nose; and, consequently, as the large single lens pro¬ 
duces only a single image at its focal point, so should its two 
portions eccentric produce only a single image at the same 
point. 

The optical center of a lens is at the central part of the 
lens, at which point both surfaces of the lens are parallel, and, 
consequently, rays passing through this point do not suffer 
any refraction at all. As we pass from the optical center the 
two surfaces of the lens begin to incline more and more, and 
rays are refracted more and more; the farther from the optical 
center that a ray passes, the more it is refracted. 

Tinted glass, which is sometimes used for spectacle lenses, 
presents a varying density of tint in different parts, correspond¬ 
ing to the varying thickness of the glass in its different parts. 
In the case of concave glasses, which are thinnest in the 
center, this is perhaps rather an advantage than otherwise; 
while in the case of convex glasses, which are thickest at the 
center, the deepened tint at this point amounts almost to a pro¬ 
hibition of their use. In this latter case, however, the inequal¬ 
ity of the tint may be avoided by making use of a plano-convex 
lens, to the plane surface of which a thin plate of tinted glass is 
cemented by means of Canada balsam. Amber has also been 
used as a material for spectacle lenses. 

Tinted glasses with parallel surfaces (that is, without any 
refractive power) are in common use to temper the light which 
reaches the eye; they are made either with plane surfaces like 
window-glass, or with curved surfaces like watch-glasses, 
these latter being coquilles. The favorite color for tinted 
glasses was formerly green, on account of its similarity to the 
color of the grass and the foliage of the trees. Green was 
gradually superseded by blue (the color of the sky), which is 
still much used. But those in most common use are a neutral 
tint, called London-smoke, and are preferable to either green 
or blue. Glasses of an amber tint are also 1 kept in the shops; 
they are known as “shooting spectacles,” and are ground to a 
dull surface except at the central portion. Amber-colored 


LENSES. 


61 


glasses, and also red glasses, by excluding all but the least 
refrangible rays of the spectrum, may serve (in a strong light) 
to improve the outlines of the retinal image in very low grades 
of myopia; blue reading-glasses, on the other hand, may 
render some slight degree of aid in low grades of hyperme- 
tropia and of presbyopia. Tinted glasses should, as a rule, 
be mounted in large, oval settings, so as to cover the entire 
front of the orbit; the coquille form of glass affords more per¬ 
fect protection than a glass with plane surfaces. London- 
smoke coquilles are of great use to persons who are exposed 
to very strong light reflected from sand, or from snow, or from 
the surface of water. London-smoke glasses, of so dark a 
color as to appear quite black in ordinary light, are used in the 
protective spectacles worn by workmen employed about elec¬ 
tric lights. 

Perhaps a word might be spoken here of one of the disad¬ 
vantages of the coquille form of smoke glasses, and that is 
their liability to have a concave focus, and it is often very 
objectionable. If the two surfaces of the glass were exactly 
parallel in every part, there could be no focus; but it can be 
understood at a glance that it is much more difficult to make 
the two surfaces parallel in coquilles than it would be in plane 
glasses, and, as a result, persons who are supposed to be wear¬ 
ing plane smoke glasses are really looking through concaves, 
which often results in straining the eyes, especially if the 
person be hypermetropic. 

Spectacles afford, to a greater or less extent, protection 
against mechanical injury, and in some certain trades it is only 
by the use of such special protectives that the liability to grave 
accidents to the eyes can be averted. Millers have long been 
in the habit of wearing large spectacles, fitted with thick win¬ 
dow-glass, when employed in the dangerous work of dressing 
millstones. Protective spectacles of mica are especially to be 
recommended for miners, quarrymen, stone-cutters, boiler¬ 
makers, and others engaged in similar dangerous employ¬ 
ments. Goggles of finely-woven wire gauze, generally with 
the fronts glazed with tinted, plane glass, are used by railway 
travelers and others as a protection against flying sparks; 
goggles made of glass bent to a cylindrical curve of about six 


62 


LENSES. 


inches radius, and furnished with cloth-covered rims to fit 
closely around the margins of the orbits, are sometimes used 
as a protection against dust in driving. 

Coquille spectacles and eye-glasses, colorless and tinted, 
are made also in the Meniscus form (with positive focus) and 
in the concave-convex form (with negative focus). Owing to 
the shorter radius of curvature of the concave surface turned 
toward the eye, these glasses are more perfectly periscopic 
than those commonly sold under that name. 

Besides being mere protectives, as just mentioned, the 
purpose of spectacles and eye-glasses is principally to supple¬ 
ment impaired accommodation (as, for instance, the convex 
glasses used in presbyopia and in accommodative paresis and 
paralysis), to relieve the accommodation of an excessive 
burden by supplementing deficient refraction (as, for instance, 
the convex glasses used in myopia), and to correct asymmet¬ 
rical refraction (as, for instance, the convex and concave cylin¬ 
drical glasses used in astigmatism). These several effects are, 
moreover, often variously combined, as in the use of strong 
convex glasses in reading (by hypermetropes with defective 
accommodation), of partially correcting concave glasses, or 
perhaps of weak convex glasses in reading (by myopes with 
defective accommodation), and of glasses of asymmetrical re¬ 
fraction (in compound and mixed astigmatism and in pres¬ 
byopia or other accommodative defect occurring in connection 
with astigmatism). 

Objects viewed through convex glasses appear larger 
than do the same objects when their images are focused by the 
-exercise of the accommodation, and, conversely, objects viewed 
through concave glasses under accommodative tension appear 
smaller than do the same objects when viewed without the 
exercise of the accommodation. Hence a presbyope using 
convex glasses in reading sees the print not only clearer than 
without glasses, but also larger than it appears to an emme- 
trope under normal exercise of the accommodation. So, also, 
an hypermetrope wearing convex glasses constantly sees all 
objects larger than when he views them without glasses, and a 
myope using concave glasses in reading sees the print smaller 
than when he reads without glasses. In hypermetropia, how- 


LENSES. 


63 


ever, the actual size of the retinal image is smaller, and in 
myopia it is larger, than in emmetropia; and it is a fact now 
well established that both hypermetropes and myopes wearing 
neutralizing glasses at the usual distance from the eye, see 
objects of about the same apparent dimensions as does an em- 
metrope without glasses. 

A convex spectacle lens is increased in effective power by 
moving it farther from the eye, and, conversely, a concave lens 
loses in effective power with any increase in its distance from 
the eye. The correct rule of practice is to mount the glasses 
as near as possible to the eyes, allowing sufficient room for the 
free play of the eyelashes. A distance of about one-half inch 
from the vertex of the cornea fulfils this condition in most 
cases, and at the same time allows the correcting lens to be 
placed almost exactly at the anterior principal focus of the eye, 
in which position of the glass the retinal image, whether in 
hypermetropia or in myopia, becomes practically equal in size 
to the image of the same object when focused by an emme¬ 
tropic eye. Whenever an hypermetrope inclines to remove 
his convex glasses to a greater distance from the eye than one- 
half inch, it may be assumed that the glasses are somewhat too 
weak to fully correct his hypermetropia, and, conversely, when 
a myope inclines to wear his concave glasses at a greater dis¬ 
tance from the eye than one-half inch, it may be assumed that 
the glasses are too strong. This particular mode of correcting 
the effect of badly-selected glasses in distant vision is but 
rarely adopted, except in the presence of defective accommoda¬ 
tion, as, for instance, by elderly hypermetropes or myopes, and 
especially by persons who have undergone an operation for 
cataract. In presbyopia it is a not uncommon practice to slip 
the convex reading-glasses far down toward the tip of the 
nose, in order to make a weak glass do the office of a stronger 
glass in improving the distinctness of the print, and also in 
increasing its apparent size; in this position of the glasses it is 
also easy to look over them at distant objects. 

The increase or diminution in the apparent size of objects 
viewed through a convex or concave lens is not uniform in all 
parts of the visual field, but is notably greater at its periphery 
than at its center. Thus a large object viewed centrally 


6 4 


LENSES. 


through a spherical convex lens is seen more highly magnified 
in its peripheral than in its central portions; a square, for ex¬ 
ample, whose angles are more distant from the center than is 
the middle of each side, is seen as if bounded by curved lines 
with their convexity turned toward the center of the field. 
The same square when viewed through a concave lens is seen 
diminished in size, but most diminished in its peripheral por¬ 
tions, so that it appears as if bounded by curved lines with 
their concavity turned toward the center of the field. 



The above figures illustrate at 2 and 3 the distortion 
under which a large square figure (1, a window, for example) 
is seen through a convex and a concave lens respectively. It 
will be observed that the distortion of the smaller squares 
increases from the center toward the periphery of the field. 

It has been often remarked that myopes, in selecting con¬ 
cave glasses, are very apt to err by making choice of glasses of 
somewhat too short focus, which cause objects seen through 
them to appear very sharply outlined. This phenomenon 
appears to be the result of the chromatic aberration of the eye, 
causing the object, when viewed through a concave glass 
under a full correction for the most highly refrangible rays 
(violet) of the spectrum, to be seen as if bounded by a very 
narrow red border, instead of by a broader violet fringe as 
when the eye is focused for the least refrangible rays (red). 
If a distant point of light be viewed by an over-corrected my¬ 
opic eye through a piece of cobalt-blue grass the light will 
appear blue with a narrow red halo; if the eye is under-cor¬ 
rected by its concave glass the light will appear red, with*a 
broader violet halo. 



















LENSES. 


65 


A convex or concave cylindrical lens, as used for the cor¬ 
rection of astigmatism, simply elongates or shortens the retinal 
image in a direction at right angles to the axis of the lens; thus 
the relation of the two diameters of the object appears altered, 
a circle appearing elongated or shortened to an ellipse, etc. 
The distortion from this cause in regular astigmatism, when 
the direction of the two principal meridians happens to be 
asymmetrical in the two eyes, may give rise to such difference 
in the two retinal images as to evoke a great variety of stereo¬ 
scopical illusions from the fusion of the two impressions in 
binocular vision; illusions of this kind are, however, very soon 
corrected by experience, as the wearer of the cylindrical 
glasses becomes accustomed to the new conditions. 

Incidental to the action of concave and convex spectacles 
in modifying the exercise of the accommodation is the effect 
which they exert upon convergence as associated with accom¬ 
modation. Convex glasses, by relieving the accommodation of 
a part of its load (as in hypermetropia), exert at the same time 
a positive effect in diminishing the convergence which stands 
in correlation to it; and consequently they (convex lenses) 
rank first among the therapeutic agents at our disposal for 
arresting the development of convergent strabismus, and even 
in many cases for its cure. Concave glasses, on the other hand, 
by increasing the demands made upon the accommodation in 
ne^r vision (as in myopia) evoke also increased action of the 
internal recti muscles with a corresponding relaxation of the 
external recti, and thus afford relief in many cases of muscular 
asthenopia and of crossed diplopia, and even of divergent 
strabismus. 

Spectacles, whether convex or concave, may be so 
mounted as to exert also a direct action upon convergence, by 
what is known as decentering —that is, by mounting the lenses 
so that the optical center of the lens is not directly in front of 
the pupil. This effect may be developed accidentally, as a 
result of imperfect centering of one or both glasses, and may 
then be attended with more or less harmful consequences; or 
it may be produced designedly, and applied with advantage to 
the treatment of muscular insufficiency. 


66 


LENSES. 


DECENTERED LENSES. 

A decentered lens acts as a sphero-prism or a cylindro- 
prism when the rays of light pass through any point except its 
optical center; and the farther removed fiom this center, the 
greater the prismatic effect produced. The distance from the 
point looked through to the optical center is the measure that 
the lens is decentered. 

In the use of prisms, when their bases are turned toward 
the nose, they relieve the internal recti muscles; in order to 
produce the same effect with decentered lenses, the thickest 
part of the lens must be nearer the nose than it would ordi¬ 
narily be. Convex lenses, in which the optical center is at the 
thickest part, are therefore carried toward the nose; or, in 
other words, the lenses are “decentered inward.” Concave 
lenses, in which the optical center is at the thinnest part of the 
lens, are for the same reason carried away from the nose 
toward the temple; or, in other words, such lenses are decen¬ 
tered outward. 

Where lenses are to be used only for reading or other 
close work, and where there is a constant effort required from 
the convergent muscles, they may be decentered as described 
above (the convex in and the concave out) without any result¬ 
ing harm, and perhaps with some advantage. 

But if such decentered lenses are of advantage when 
specially prescribed, they produce a greater amount of harm 
when not required; and unless there is some reason for decen¬ 
tering a lens, and unless it is decentered in the particular way 
the person requires, the optical center should be at the place 
where the line of sight will pass through it, which may be just 
at the center in distance glasses and a little to the inner side in 
reading glasses. 

It is important not only that the optical center should not 
be too far in or out, but that it should be at the proper level, 
and that the two optical centers (of the two lenses composing a 
pair of spectacles) should both be on the same level. If one is 
higher or lower than the other, the prismatic effect will be 
produced on the superior and inferior recti muscles, causing 
one eye to look more up or down than the other, and, the eyes 


LENSES. 


67 


naturally being intended to look on the same level, such devia¬ 
tions may give rise to considerable inconvenience and discom¬ 
fort, and thus entirely defeat all the benefit that might otherwise 
be derived from a carefully adjusted pair of lenses. Such 
improper decentering is commonly found in cheap spectacles. 
We sometimes see an individual wearing his glasses with one 
side higher or lower than the other; may not such a person be 
trying to overcome the discomfort produced by such an im¬ 
properly decentered pair of glasses? Every pair of lenses 
should be carefully tested to ascertain if the optical centers 
are properly placed. 

HOW TO FIND THE OPTICAL CENTER OF A LENS. 

The best way to locate the optical center of a lens is to 
look through the lens at a straight line on a card, the line being 
long enough to be seen through the entire lens, and also above 
and below it. If you look at the line through the lens at any 
part except its optical center, the part of the line seen through 
the lens will not seem continuous with the parts of the line 
zseen above and below it. 




The above illustrations show this interruption of the line 

_figure I showing it as it occurs in a convex lens, the portion 

seen through the lens seeming to be carried away from the 
optical center, while figure 2 shows it as it occurs in a concave 
lens, where the portion of the line seen through the lens seems 
to be carried toward the optical center. But if you look at the 








68 


LENSES. 


line through the optical center of the lens, it will appear as one 
continuous line above the lens, through it and below it. Hence 
the rule is to move the lens until you make the line continuous, 
when you will draw a line with ink directly across the face of 
the lens, exactly over the line seen through it, as shown in the 
first figure in the following diagram: 




Having now determined one line, C D, in .the above figure 
that is continuous, we know that it passes through the optical 
center, and we then turn the lens so that this line C D shall be 
at right angles to its former position. We then repeat the 
same steps as before, moving the lens until we have a straight, 
continuous line above, through, and below the lens, which we 
mark with ink, as before, when we know we have another line 
that passes through the optical center of the lens. Now, (the 
intersection of these two lines is the point of location of the 
optical center of the lens. 

Instead of cutting out two spectacles lenses from a pheri- 
pheral zone of a very large lens (as in the illustration of ortho- 
scopic lenses on a previous page) it is found to be quite as 
convenient and much less expensive to grind the required 
convex or concave focus upon one side of a prism, while upon 
the other surface of the prism a cylindrical lens may be ground 
if required for the correction of any existing astigmatism. 
Such glasses are sometimes used in correcting optical defects 
when they are complicated by insufficiency of the internal or 





LENSES. 


69 


external recti muscles, and would thus come into advantageous 
use in many cases of muscular asthenopia, and of diplopia of 
moderate grade, even when dependent upon paralysis of one 
of the recti muscles; but in well marked cases of strabismus 
they are seldom of any benefit. 

The decentering of a spherical lens, either convex or 
concave, necessarily gives rise ito some distortion of the retinal 
image, and the greater the decentering the more pronounced 
will be the distortion. This is shown in the figures on page 
64, where the small square at the sides and the larger square 
at the angles are drawn as they are seen through peripheral 
portions of a convex or a concave lens. Besides this, the 
several pencils of rays, after refraction by a decentered lens, 
are no longer homocentric (homocentric is derived from two 
Greek words, and means “having the same center”); that is, 
they no longer converge toward or diverge from a focal point, 
but pass through two so-called focal lenses. In other words, a 
decentered spherical lens always gives rise to some degree of 
astigmatism, which in some cases may be so great as to require 
correction, which can be done by using lenses of a plano¬ 
convex or a plano-concave form, and grinding a cylindrical 
surface upon the plane side. 

When prismatic glasses with plane surfaces are mounted 
with their bases inward or turned toward the nose, they relieve 
the internal recti muscles of a part of their work in converg¬ 
ence, and may also restore binocular vision at a distance in 
cases of crossed diplopia, dependent on very slight divergence 
of the visual axes. When they are mounted with their bases 
outward, or turned toward ithe temple, they are applicable in 
some cases of insufficiency of the external recti muscles and of 
preponderance of the internal recti, as in the homonymous 
diplopia which is sometimes observed in low grades of con¬ 
vergent strabismus. A prismatic lens, mounted with its base 
up or down before one eye only, may be used to neutralize the 
effect of a slight upward or downward deviation of the eye 
before which it is worn, or the correction may be divided 
between the two eyes by making use of two prisms—one 
mounted with its base upward and the other with its base 
downward. Prisms are not usually worn stronger than eight 


70 


LENSES. 


degrees, on account of the conspicuous colored fringes due to 1 
chromatic dispersion. 

The proper effect of any spectacle lens can only be ob¬ 
tained, as has already been shown, when the lens is accurately 
centered in front of the pupil; and another important condition 
is that the plane in which the lens is set shall be perpendicular 
to the direction of the visual axis. The distance between the 
centers of the two lenses of a pair of spectacles intended to be 
worn for distant vision should therefore be exactly equal to 
the interpupillary distance, and in the case of reading-glasses, 
where the visual axes are made to converge toward the printed 
page, the distance between the centers of the two lenses should 
be somewhat less than the interpupillary distance. 

Furthermore, the two lenses should be set in one and the 
same plane, perpendicular to the direction of the visual axes;, 
for example, vertical in the case of glasses to be worn for dis¬ 
tant vision, while they should be tipped forward in spectacles 
which are to be used for reading. Strictly speaking, the lenses 
of reading-spectacles should also be inclined a little toward' 
each other, so as to maintain the perpendicularity of the lenses 
to the visual axes when they are directed toward each other as 
in the act of convergence. 

Whenever a convex or a concave spherical lens is set 
obliquely to the direction of the visual axis, its refractive power 
is increased, though in a very different degree, in all its me¬ 
ridians; the increase being greatest in the meridian corre¬ 
sponding to the plane of the arc through which the lens is 
rotated, and least in the meridian of the axis about which the 
rotation takes place. In the case of a convex or a concave 
cylindrical lens rotated about its axis, the increase in refractive 
power varies from a maximum in the meridian at right angles 
to the axis, to zero in the meridian of the axis. When rotated 
about the meridian at right angles to its axis, a convex or con¬ 
cave cylindrical lens shows also a positive increase in refractive 
power, though in a lesser degree than when it is rotated about 
its axis. 

It follows that a tipped spherical lens becomes practically 
equivalent to a (somewhat stronger) spherical lens with a 
cylindrical lens added to it, and that in the case of a sphero- 


LENSES. 


71 


cylindrical lens the special effect of the cylindrical surface may 
be either increased or diminished, according as the compound 
lens is rotated about one or the other of its principal meridians. 
A tipped concave spherical lens may be occasionally utilized in 
distant vision in myopia, with astigmatism of a low grade, 
when the ocular meridian of greatest refraction happens to be 
vertical or very nearly vertical, and a vertically-mounted con¬ 
vex spherical lens may be given for reading when the ocular 
meridian of greatest refraction happens to be exactly or nearly 
horizontal. A familiar instance of such a correction in myopia 
is seen in the not-infrequent preference given to a tipped con¬ 
cave spherical eye-glass over glasses mounted in a vertical 
position in a spectacle frame. So, also, after an operation for 
cataract, a sphero-cylindrical lens, with axis horizontal, may be 
required to raise distant vision to its maximum, although for 
reading a spherical glass may be preferred, on account of the 
augmented refraction in the vertical meridian incident to the 
downward direction of the visual axes. 

Again, in myopia with astigmatism, when the ocular 
meridian of greatest refraction happens to be approximately 
horizontal, the wearer of concave spherical glasses may learn 
the trick of looking obliquely through his glasses, either to the 
right or to the left, and may thus add materially to his acute¬ 
ness of vision, though at the cost of acquiring an awkward 
carriage of the head. So, also, an hypermetrope with some 
degree of astigmatism, when the ocular meridian of greatest 
refraction happens to be approximately vertical, may similarly 
get a better correction from his convex spherical glasses by 
looking obliquely through them to one side. 

A myope, wearing concave glasses of a power insufficient 
to fully correct his myopia, is very apt to look obliquely to one 
side in order to improve his vision for the vertical lines of a 
distant object, and he may at the same time contract the open¬ 
ing of the eyelids in order to improve his vision for horizontal 
lines. In hypermetropia this habit is but rarely acquired, for 
the reason that here the accommodation is generally brought 
into exercise to supplement the effect of the glasses, but in 
aphakia, owing to the total loss of accommodative power, it is 
oftentimes made use of. 


72 


LENSES. 


Spectacle lenses are usually mounted in oval rings of 
metal, or, in the case of eye-glasses, of tortoise-shell, horn, 
hard rubber, celluloid, etc. The rims used in mounting convex 
glasses are almost always grooved, so as to grasp the sharp 
edge of the lens; concave glasses are also usually ground to a 
sharp edge and mounted in grooved rims, but it is also a not 
infrequent practice to groove the lens itself and to sink the rim 
in the groove, the rim in such a case being made of thin steel 
or gold wire. Convex lenses may also be mounted in this way, 
with the rims sunken, but with the great objection of making 
the lens needlessly thick and heavy. The so-called frameless 
or rimless glasses have the bridge and temples attached by 
means of screws passing through holes drilled in the lenses at 
their nasal and temporal sides; concave lenses, with their thick 
margins, lend themselves better to this construction than do 
convex lenses. 

Caprice and fashion have sometimes dictated the wearing 
of a single eye-glass, carried at the end of a riding-whip or a 
fan, or worn suspended by a cord; in the latter case the glass 
is held in front of the eye by a contraction of the orbicularis 
muscle upon its rim in a singularly awkward and inconvenient 
fashion. This is known as a monocle (one eye). 

Binocular (two eyes) glasses may be divided, according to 
the way in which they are held before the eyes, into three 
groups: first, eye-glasses held in the hand, and which are 
known as lorgnettes; second, those held in place by means of a 
spring which pinches the nose, the French pince-nez; and 
third, spectacles proper, which are held in place by means of 
temples or side-pieces, passing above and behind the ears. To 
these three principal types may be added a fourth, now disused 
except in the case of protective goggles, in which the glasses 
are held in place by means of tapes or an elastic band passing 
around the head above the ears. 

The several parts of a pair of spectacles are (i) the rims in 
which the glasses are mounted; (2) the bridge, or nose-piece, 
by which the rims are connected and supported upon the 
bridge of the nose; and (3) the side-pieces, or temples, by 
which the spectacles are held in place upon the head. The 
size of the rims and the length of the bridge should be so pro- 


LENSES. 


73 


portioned as to conform both to the interpupillary distance, in 
order that the wearer may look through the centers of the 
glasses, and to the width of the face, so that the side-pieces 
may touch but not press too tightly against the sides of the 
head. 

In fitting spectacle frames, in the case of great width of 
the face, larger rims are generally required than when the face 
is narrow; any variation from the usual proportion between 
the interpupillary distance and the width of the face may 
generally be met by varying the length of the bridge. In 
order to properly fit the face, it may sometimes be necessary to 
select a frame in which the distance between die centers of the 
rims is a little greater or a little less than the interpupillary dis¬ 
tance, but in such a case care should be taken in setting the 
glasses to preserve the proper distance between the centers. 

The bridge should be shaped to fit the nose and partially 
to encircle it; noses, however, differ very greatly in promi¬ 
nence and thickness, so that no single type of bridge is suited 
to all cases. The “hoop” bridge is one of the older forms, and 
is suited to noses of considerable thickness and prominence; 
the hoop may lie in the same plane with the glasses, or it may 
be turned forward at any desired angle. The modified form is 
now more commonly used than the plain hoop, and, like it, 
may be set at any required angle to the plane of the glasses. 
These forms are well suited to noses of some prominence, but 
in other cases they fail to support ithe glasses at the proper 
height, or at a sufficient distance from the eyes to avoid contact 
with the eyelashes. To obviate this serious defect in the hoop 
bridge, two very useful modifications have been recently intro¬ 
duced, viz., the twisted (or “snake”) bridge, and the “saddle” 
bridge. The so-called X bridge and the K bridge have been 
extensively used in frames of very light weight; they are, how¬ 
ever, of less general applicability than the other forms. 

A perfect bridge should present a rather broad surface of 
contact with the nose, and special care should be taken to 
secure an accurate and easy fit; in some rare cases it might 
even be worth while to make a cast of the nose upon which to 
mould the bridge. Bridges of the X and K patterns, as 
usually made from thin wire, are apt to cut the nose. A gold 


74 


LENSES. 


bridge is often to be preferred, even when the other parts of 
the frame are made from steel, as being more easily moulded 
to the nose and free from liability to rust; a recent improve¬ 
ment (if improvement it can be called, as it is open to many 
objections) consists in the application of a lining of fine cork 
to the hoop or saddle bridge. Frames of tortoise-shell were 
once in common use, and are, on many accounts, to be com¬ 
mended; frames are still made in limited quantities with the 
rims and bridge formed from a single piece of shell or of cellu¬ 
loid. Either material may be softened by the application of a 
gentle heat, so that the bridge may be bent to almost any 
desired shape. 

The side-pieces, or temples , should be slightly bowed to fit 
the sides of the head; they should be of at least sufficient 
length to reach a little behind the ears, as in the single temples, 
or they may be made a couple of inches longer by jointing 
them on a pivot at about the point where they rest upon the 
ears, as in the turn-pin temples; or by the use of a sliding 
mechanism, as in the sliding temples. But best of all are the hook 
or riding-bow temples, which are made of thin and very elastic 
wire bent downward in an easy curve behind the ears. By a 
recent invention, consisting in the introduction of delicate, 
spirally-wound wire in the side-pieces, near their proximal 
ends—that is, near the joint—the flexibility of hooked temples 
has been increased. 

The several parts of a spectacle frame should be nicely 
proportioned to each other. In the case of spectacles with 
single temples, the bridge should be of sufficient stiffness to 
maintain its shape unaffected by the lateral spring of the side- 
pieces. Only when hooked temples are used is it advisable to 
make all parts of the frame of very light weight. 

Fashion has played its part in prescribing the form of 
spectacle lenses; the original shape was doubtless circular, a 
form still occasionally seen in spectacles, but more frequently 
in eye-glasses. The shape now generally preferred is a nearly 
regular oval, but with considerable variation in the proportion 
of the two principal diameters. Another form is oblong with 
rounded angles. A parallelogram, with the four corners cut 
off by straight lines (octagon glasses), was a favorite shape 


LENSES. 


75 

not many years ago, and, although rather ungraceful, is still 
occasionally used. 

Pantoscopic glasses, so-called, have the upper part of the 
rims flattened, in order to permit the wearer of reading-glasses 
to see over them in looking at distant objects; the lenses are 
also set, as a rule, obliquely to the direction of the side-pieces,, 
but perpendicular to the visual axis in reading. 

In some forms of ametropia, with deficient accommoda¬ 
tion, it is often convenient to mount two half-lenses in each 
rim, the upper half (convex or concave) of a focal length suited 
to the correction of the actual hypermetropia or myopia; the 
lower half, of the focus needed for reading. A similar effect is 
obtained, though somewhat less perfectly, by grinding the 
upper and lower halves of the same lens to different radii of 
curvature (bi-focal lenses). Benjamin Franklin is said to have 
worn double focus lenses, the upper half concave to correct his 
myopia, and the lower half convex to correct his presbyopia. 

Recently several improvements have been made in the 
double-focus glasses; in one case a crescent-shaped piece is cut 
from the lower part of the lens, and replaced by a similarly 
shaped piece of lens of the required focus for reading. In the 
other case, in the so-called lenticular glasses, the necessary 
extra strength required for reading is added in the shape of a 
convex lens, ground very thin and cut in the shape of a cres¬ 
cent, and cemented firmly on the lower portion of the lens. 

Another useful arrangement consists in mounting the 
reading correction in a separate frame (extra fronts) to be 
hooked upon the front of the spectacles that are worn for 
constant use. 

Eye-glasses of the pince-nez pattern have been used since 
an early period in the history of spectacles, but their construc¬ 
tion has been greatly improved within the past ten or twenty 
years. In an old painting, in one of the European galleries, 
an elderly bishop is represented as reading through a pince-nez 
set very low upon the nose; the rims of the glasses are circu¬ 
lar, and the connecting piece seems perfectly rigid. This rigid 
construction and the manner of wearing them low down upon 
the nose explain the objection formerly made to them, as liable 
to compress the nostrils and interfere with the breathing, and 


76 


LENSES. 


so impart a nasal quality to the voice. A facsimile of that 
portion of the painting containing this head has lately been 
reproduced in color, in one of the publications of a London 
Society. 

In the older forms of eye-glasses, the centers of the glasses 
stand much too near together, and the glasses themselves are 
very apt ito tip forward in a way that is sometimes very detri¬ 
mental to the optical effect intended to be obtained from them. 
In many cases, also, they stand so near to the eyes as to allow 
insufficient room for the play of the eyelashes, and whenever 
the nose is unsymmetrical one glass is sure to stand higher 
than the other. 

In the recent eye-glasses of improved construction these 
defects are, to a noticeable extent, obviated. Thus most of the 
modern eye-glasses have some form of projecting nose-clips, 
which may be set either in the same plane with the glasses, or 
in a plane behind that of the glasses, and inclined to them at 
any required angle to secure the best possible bearing upon 
the sides of the nose; some eye-glasses have also a provision 
for adjusting the clips upon the two sides, so as to fit noses of 
almost any shape and thickness, and of very considerable 
degrees of asymmetry. A cork lining to the clips possesses 
many advantages—in softness to the skin and in holding 
glasses in position. The tilting of the glasses, in cases of 
exceptional prominence of the forehead, can, in a great 
measure, be obviated by giving to the conneating spring a 
decided forward slant. 

All that has been said regarding the position of the glasses 
when mounted in spectacle frames applies equally to eye¬ 
glasses, and it is often possible, by taking sufficient pains with 
modern patents of eye-glasses, to fit up a pince-nez which shall 
prove a tolerable substitute for a pair of spectacles. 

For mounting cylindrical and prismatic lenses, eye-glass 
frames are especially unsuitable, by reason of the difficulty 
which is apt to be experienced in keeping them straight before 
the two eyes, although even this difficulty is, to a great extent, 
obviated by the recent patent forms of guards. The especial 
convenience of eye-glasses lies in the fact that they are easily 
put on and off. They are not so well suited for reading 


LENSES. 


77 


except for a few minutes at a time, nor for continuous wearing 
in distant vision. In many cases of asthenopia, and in pro¬ 
gressive myopia especially, the wearing of eye-glasses should 
be prohibited. 

The different methods used in testing eyes for the correc¬ 
tion of the several refractive and accommodative defects, 
whether simple or complicated, will be fully described under 
their proper head. Suffice it to say, at this time, 'that the 
points to be investigated in all cases are: 

1. The state of the refraction. 

2. The acuteness of vision. 

3. The state of the accommodation. 

Only after these three determinations have been made 
with at least approximate accuracy, can the selection of glasses 
for any particular kind of work be made intelligently. In the 
present state of diffusion of knowledge in the domain of phys¬ 
iological optics, these tests can be safely intrusted only to the 
ophthalmic specialist, such as have given the subject special 
study and taken a course with an optical college; spectacle 
dealers and jewelers, and even the general practising physi¬ 
cian, being alike incompetent, as a rule, to decide any but the 
simplest questions. 

A person who has arrived at the age of forty-five years 
without having experienced any trouble in the continuous use 
of his eyes in near and distant work may not be likely to 
commit any great error in buying weak convex glasses when 
he becomes conscious that he is suffering from the disabilities 
of presbyopia; but even in such a case an examination of the 
eyes by a competent observer may bring to light some 
measure of astigmatism which it may be well worth while to 
correct, or possibly some pathological condition which it may 
be of vital consequence to detect in its incipiency. The indis¬ 
criminate selling of concave spectacles and eye-glasses to 
young myopes, or to young people hastily assumed to be 
myopic, is a most reprehensible practice, although it is, un¬ 
fortunately, an almost universal one. 


.78 


LENSES. 


CHROMATIC AND SPHERICAL ABERRATION. 

The optician frequently sees and hears the terms “chro¬ 
matic and spherical aberration,'” and it is important that he 
should have a clear understanding of what is meant by them. 
An extended intercourse with opticians has shown that a 
majority of them do not have the slightest idea of what is 
meant by these terms, while others have a vague and incorrect 
idea about them. The answers given to these questions by 
opticians are oftentimes most laughable—so very far from the 
real meaning are they. 

Aberration .—Aberration of light is a peculiar phenome¬ 
non, which causes an alteration in the apparent position of a 
star from its true place in the heavens; this deviation being but 
slight in the course of a year. It is due to the fact that the 
observer is carried along by the motion of the earth in its 
orbit, while the light is traveling from the star to the earth, and 
the velocity of the earth’s motion is a measurable quantity in 
relation to the velocity of light. The aberration of light is, 
therefore, due to the combined effect of the transmission of 
light and of the earth’s motion. The effect of this combination 
of motions may be best explained by a familiar illustration. 

Suppose the raindrops were falling (in a perfect calm) per¬ 
pendicular to the earth’s surface, and the observer were stand¬ 
ing on a platform-car, on a railroad track, and rapidly moving 
forward and backward; the drops would strike him at an 
angle deviating from the perpendicular in proportion to the 
swiftness of his motion. The direction of this deviation would, 
in either case, be toward the side to which he is moving; and 
this is just the case with the light coming to us from the 
heavenly bodies. This becomes evident when we compare the 
direction of the raindrops with that of the light, and the 
motion of the car with that of the earth in its yearly orbit. It 
is just as evident that if the direction in which light reaches us 
be changed, the position of the body from which the light 
proceeds must be changed also. 

Chromatic Aberration .—Lenses which form perfect images 
are very difficult of construction, and there are two main 
imperfections to be corrected—chromatic and spherical aber¬ 
ration. 


LENSES. 


79 


In studying the first of these conditions we find that in 
the image formed by a simple, ordinary lens all the outlines of 
figures are found to be slightly edged with rainbow hues. If 
we look through such a lens at an object, the outlines of the 
object will be similarly edged with colors, especially if the 
object lie near the margin of the lens. The explanation of 
this condition is as follows: 

Ordinary sunlight, as every one knows, consists of a 
number of colors mixed together, the mixture producing the 
impression of white. If a beam of sunlight be made to pass 
through a glass prism, the beam is not only bent, but the 
different colors of which the beam is composed are unequally 
bent, so that they are separated and spread out over a con¬ 
siderable space, this colored space being called the spectrum, 
and the different colors into which the white light is separated 
are, in their regular order, red, orange, yellow, green, blue, 
indigo, and violet. R 



In the above figure the ray of light A B is bent by the 
prism so as to become A C D; this is known as refraction of a 
ray of light. But each of these different colors is refracted in 
a different degree, by the lens, and they are incapable of being 
united by it in any one single focus; red is bent the least and 
violet the most, the other colors lying between these extremes, 
thus being spread out over a considerable colored space. This 
unequal refraction is known as dispersion. 

If we look through a prism at objects, we will find that 
the outlines of the objects will be edged with exactly similar 
colors. Now all refraction is accompanied by dispersion; and, 
therefore, a simple, uncorrected lens always disperses, espe- 





8o 


LENSES. 


cially on the edges where the refraction is greatest; and con¬ 
sequently the images made by such a lens will also be edged 
with colors. As the convex lenses used in telescopes, micro¬ 
scopes, and other optical instruments refract the light to 
focal points, this dispersion causes an infinite number of foci. 


S 



Chromatic Aberration. 


In the above figure the light from a radiant point A is 
refracted by the lens, and, being white light, is also dispersed; 
the violet rays, being bent the most, reach a focus at B, while 
the red rays, being refracted the least, come to a focus at C, 
and the other colors of the spectrum at intermediate points. 
There is, therefore, no point where all the rays from the light 
A come to a focus; that is, there is no common focal point for 
A. The best place for the receiving screen would be S S; but 
even here there is no perfect focus. Evidently, therefore, the 
conditions of a perfect image are not fulfilled. The question 
at once occurs: “Can this defect be corrected?”—and the 
answer is, that it is corrected in every good lens. 

In order to understand how this is done, it must be 
remembered that concave and convex lenses antagonize each 
other, and if of equal refractive power they neutralize each 
other. Therefore, a combination of a double convex and a 
double concave lens, if of same material and of equal 
curvature, will produce no refraction, because the refraction 
produced in one direction by the convex lens is completely 
destroyed by the refraction in the opposite direction of the 
concave lens. Such a combination will, therefore, make no 
image. In order that a combination of a convex and a con¬ 
cave glass should produce an image, it is necessary that the 
convexity should predominate over the concavity; this is the 
first point to be remembered. 




LENSES. 


81 


It might seem, on first thought, that the amount of dis¬ 
persion would be proportional to the amount of refraction, but 
experiments have proven that diverse refracting substances 
differ considerably in this respect, and hence we have the 
second point to be remembered, and that is, that dispersion is 
not always in proportion to refraction. Some substances have 
a higher refractive power and a comparatively low dispersive 
power, and vice versa. This is the case with different kinds of 
glass. Their dispersing and refracting properties are deter¬ 
mined by passing a ray of light through solid prisms of differ¬ 
ent material, or liquid prisms enclosed between glass plates. 
The refractive power is then measured by the amount of de¬ 
viation of the ray, and the dispersive power by the length of 
the colored spectrum produced. So it has been found that if 
the relative amounts of refraction of water, crown glass, flint 
glass, and oil of cassia are expressed by the numbers 133, 152, 
162 and 159, the amounts of dispersion or the lengths of their 
spectra are in ratio of 145, 203, 433 and 1080. If the angle of 
a prism is increased, the refracting and dispersing power both 
increase in the same ratio; and it is evident that two prisms of 
different material may be made at such angles that they pro¬ 
duce the same length of spectrum or possess the same disper¬ 
sion, but that then their refracting powers will not be the same. 

Now suppose we select a glass with excess of refractive 
over dispersive power for our convex lens, and one with excess 
of dispersive over refractive power for our plano-concave lens, 
and cement these together as a compound lens. It is evident 
that they may be so related that the plano-concave lens shall 
entirely correct the dispersion of the convex lens without 
neutralizing its refraction, and, therefore, the combination will 
be a refractive but not a dispersive lens, and, therefore, will 
produce a pure white spot without colored edges. Such a 
compound lens is called achromatic. 

This is the principle on which art makes achromatic 
lenses, and all our modern telescopes, microscopes, photo¬ 
graphic and good optical instruments have their lenses thus 
corrected. 

It is an interesting historic fact that the hint for the cor¬ 
rection of chromatism by combination of lenses was taken 


82 


LENSES. 


from the structure of the eye by Enler, and afterward carried 
out successfully by Dollond. That the chromatism of the eye 
is substantially corrected is shown by the complete absence 
of colored edges of strongly illuminated objects, and the sharp 
definition of objects seen by good eyes. 

A convex lens of crown glass brings the rays together to 
a number of differently-colored foci, of which the red rays will 
be farthest from the lens. A concave lens will throw the red 
rays nearest the axis; and if this concave lens is made of flint 
glass (a material having a slightly greater refracting power 
and a much greater dispersive power), and ground to such a 
curve as completely to neutralize the dispersion or coloring of 
the first lens, while it affects its refraction only so far as to 
lengthen its focal distance, the combination will bring the rays 
to a focus, without separating the luminous rays into their 
colored constituents. Such a lens is said to be corrected for 
chromatic aberration. Sometimes the concave correcting lens 
of flint glass does not quite accomplish the purpose, and the 
combination is said to be under-corrected; but sometimes the 
opposite is the case, when the combination is said to be over¬ 
corrected. 

The lenses of the eye are also apparently corrected in a 
similar manner. The eye (or rather the refractive media of the 
eye) consists of three lenses—the aqueous, the crystalline and 
the vitreous. These have curvatures of different kinds and 
degrees—the aqueous is convex in front and concave behind, 
the crystalline is bi-convex, and the vitreous is concave in 
front. As the convex outer surface of the vitreous cannot be 
regarded as a refracting surface, since it is in direct contact 
with the screen to be impressed, it may be considered as a 
plano-concave lens. The refractive powers of the material of 
these are also different—that of the crystalline being greatest 
and the aqueous least. Their dispersive powers have not been 
determined, but they probably differ in this respect also. 
Thus, then, we have here also a combination of different 
lenses, of different curvatures, and different refractive and 
probably different dispersive power, and all for the same pur¬ 
pose, and that is the correction of chromatism. Then, too, we 
are not conscious of this defect for another reason, and that is 


LENSES. 


83 


because the eye is adjusted for the middle and brightest por¬ 
tion of the spectrum, viz., yellow and green to blue, while the 
red and violet rays form circles of diffusion around the image, 
which blend, and thereby diminish in distinctness of color; 
being, moreover, much less bright than the sharply-defined 
image which they surround. 

But we can see these colored circles of diffusion quite dis¬ 
tinctly on looking at some bright object, while covering one- 
half of the pupillary aperture with an opaque screen, for by 
thus cutting off one-half of the pencil of light we prevent the 
blending of the color-rings, and the object shows, then, a blue 
edge on one side and a yellow margin on the other. The 
chromatic aberration is also quite noticeable on observing a 
luminous point through a violet glass, which absorbs the 
middle part of the spectrum, whereupon we see either a red 
point surrounded by a blue halo, or a red margin around a blue 
point of light, according to the refractive state of the eye. So 
that, by close observation and refined methods, we must admit 
that it can be shown that the chromatism of the eye is not per¬ 
fectly corrected after all; it can be observed in the extreme 
colors, red and violet, but its degree is so small as not to inter¬ 
fere at all with the accuracy and clearness of vision. 

SPHERICAL ABERRATION. 

We come now to the consideration of another defect, 
much more difficult of correction, and which is known as 
spherical aberration , or simply aberration , in contradistinction 
to chromatic aberration, which is known simply as chromatism. 
Spherical aberration arises from the nature of the curve used 
in making lenses and reflectors. Geometry proves that par¬ 
allel rays can only be refracted and reflected to a single focus 
by a parabolic curve; but the form of lenses and reflectors 
most easily made has a spherical curvature; that is, they are 
ground as part of a sphere, which differs from a parabola in 
the fact that in the latter the amount of curvature increases 
toward the center or axis. The consequence is that a section 
of a sphere, or an ordinary spherical lens, not having curva¬ 
ture enough toward this point, has an infinite number of foci 
at different distances; that is, there is a difference in the re- 


8 4 


LENSES. 


fractive power of different portions of the same lens, the mar¬ 
ginal portions of the lens having an excess of refractive power 
as compared with the central portions, which excess of refrac¬ 
tion increases with the distance from the center; therefore the 
focal point for marginal rays is not the same as for the central 
rays—that formed by the portions of the lens nearest the axis 
will be the farthest away, while that formed by the portions of 
the lens closest to the circumference will be the nearest. 




Spherical Aberration. 


The two figures given above represent the case of this 
aberration by refraction and by reflection. In the first figure, 
illustrating the case of refraction, the marginal rays, or those 
passing near the circumference of the lens, are quickly united 
in a focus, as at E , while the central rays or those passing 
near the axis of the lens, are not brought to a focus so soon, it 
being farther away, as at F. The best place for the receiving 
screen would be between these two points, as at S S, but even 
there the image would not be sharp and clear. 

In the second figure, illustrating the case of reflection, the 
curved mirror or reflector reflects the rays falling on it near the 
circumference to a near point, as at M, while the rays falling 
on the mirror near the center will be reflected to a farther 
point, as at N. In such lenses there is no common focal point 
for all the rays, and, therefore, the conditions of a perfect image 
are not fulfilled—the image is more or less blurred. This 













LENSES. 


85 


defect should be corrected—and it is corrected—in the best 
lenses. When the aperture of the lens or mirror is small, these 
differences are practically inappreciable; but when the aperture 
must be large, as is the case with astronomical telescopes, 
peculiar arrangements are contrived, so that in making the 
lenses or reflectors a curve is obtained as nearly as possible 
of the parabolic form. 

Spherical aberration may be greatly decreased by the use 
of diaphragms, which cut off all but the central rays, but in 
this case we obtain distinctness at the expense of brightness, 
and this can only be done when the light is very intensely 
bright. Again, spherical aberration may be reduced by using 
several very flat lenses instead of one thick lens, which is the 
plan used in many instruments. But complete correction can 
only be made by increasing the refraction of the central por¬ 
tions of the lens, and this may be accomplished in two ways, 
viz., either by increasing its density, or by increasing the 
curvature of this part, and, therefore, its refractive index. It 
is by this last method that art makes the correction. By 
mathematical calculation it is found that the curve must be 
that of an ellipse. A lens, to make a perfect image, must not 
be a segment of a sphere, but of the end of an ellipsoid of revo¬ 
lution about its major axis. It is justly considered one of the 
greatest triumphs of science to> have calculated the curve, and 
of art to have carried out successfully the suggestion of 
science. 

Art has not been able to achieve success Dy the first-men¬ 
tioned method. It has not been found possible to so graduate 
the increasing density of glass from the surface to the center of 
a lens so as to correct the aberration; but it is apparently by 
this first method, or, perhaps, by a combination of both, that 
nature is enabled to correct the spherical aberration that would 
otherwise exist in the eye. The crystalline lens increases in 
density and refractive power from surface to center, so that it 
may be regarded as consisting of ideal concentric layers, in¬ 
creasing in density and curvature until the central nucleus is a 
very dense and highly refractive spherule. 

The surface of the cornea has the form of an ellipsoid of 
revolution about its major axis, and, therefore, doubtless con- 


86 


LENSES. 


tributes to the same effect. In looking at very near objects,, 
the contraction of the pupil also tends in the same direction by 
cutting off the marginal rays. But, however the result is ac¬ 
complished, whether by one or both methods, it is certain that 
in good eyes it is completely achieved, because the clearness 
of vision is wholly conditioned on the sharpness of the retinal 
image. 

It is probable that the peculiar structure of the crystalline 
lens has also another important use in the lower animals, if not 
in man. It has been shown that in a homogeneous lens, while 
the rays from radiants near the middle of the field of view (that 
is, directly in front) are brought to a perfect focus, the rays 
from radiants situated near the margins of the field of view 
(that is, of very oblique pencils) are not brought to a focus. 
Therefore the picture formed by such a lens is distinct in the 
central parts, but very indistinct on the margins. Now it has 
been shown that this defect of a homogeneous lens is entirely 
corrected in the crystalline lens by its peculiar structure; 
therefore, this peculiar structure confers on the eye the capac¬ 
ity of seeing distinctly over a wide field without changing the 
position of the point of sight. This capacity has been called 
periscopism . 

HOW TO MEASURE LENSES. 

It is often necessary for the optician to determine exactly 
the strength or number of a lens that comes into his hands, 
and every optician should be able to do this quickly and accu¬ 
rately. The first thing to be determined is whether it is a 
convex or a concave lens, and the second point for determina¬ 
tion is as to whether it is a simple or a compound lens. 

In ascertaining the first point, the lens is held up to the 
light and the observer looks through it at the window-sash or 
any stationary object; the lens is then moved to and fro before 
the eye while the sight is fixed upon the object, and if the 
object looked at appears to move in the same direction with 
the lens, it is a concave lens; while if the object appears to 
move in an opposite direction to the lens, it is a convex lens. 
An experienced observer, after a little practice, can at the same 
time give an approximate guess as to the strength of the lens 
by noticing the rapidity with which the object appears to move 


LENSES. 


87 


in one direction or the other; the stronger the lens the more 
the apparent motion, while the weaker the lens the less will be 
the apparent motion, until the lens becomes so weak as to be a 
plane lens, when there is no apparent motion at all. 

The strength of a convex lens can be exactly ascertained 
by directly measuring its focal distance. In following this 
method the lens is held in front of a window in such a way that 
the rays from outside objects will pass directly through it, and 
images of such outside objects will be formed upon a screen or 
paper placed back of the lens. Then either the screen or the 
lens is moved to and fro, nearer to or farther from each other, 
in order to ascertain exactly the distance apart they are when 
the images of the outside objects formed on the screen are the 
clearest and most distinct. This distance (that is, the distance of 
the lens from the screen) is the distance desired, and expresses 
the strength or number of the lens in inches, if an ordinary 
yard-stick or rule be used; while if the optician is accustomed 
to use the metric system, he can readily convert this inch- 
number into dioptres, according to the rule which will be 
given in the next chapter. 

In order to ascertain the strength of a concave lens, ac¬ 
cording to the same method, we combine with it a stronger 
convex lens, and then measure the focal distance of the lens 
resulting from the union of the two, which, when found, we 
subtract from the strength of the original convex lens, and the 
remainder is the strength of the concave lens desired. For in¬ 
stance, suppose the original convex lens was a + 5 D. lens, 
and after the concave lens was placed in union with it the re¬ 
sulting convex lens was + 2 D., then the strength of the con¬ 
cave lens would be — 3 D., as follows: 5 D. — 2 D. = 3 D. 
Instruments have been constructed on these principles for the 
purpose of measuring lenses, by means of which it is possible 
to ascertain the focal length of any lens in a few seconds, by 
reading it directly from the scale attached to the instrument. 

Neutralization .—Measuring lenses by neutralizing them 
is, however, the most commonly used method and the most 
convenient. This method depends on the finding of another 
lens that will exactly neutralize or nullify the lens desired to be 
measured; that is, make it of none effect, or as a plane glass. 


88 


LENSES. 


As soon as a lens comes into the experienced optician’s 
hands, his practised eye tells him in an instant if it is convex 
or concave, according to the method already described. If it 
be convex, he takes from his trial-case a concave lens of such 
strength as he thinks will nearly neutralize it, and placing the 
two together he tries the effect of the combination on the op¬ 
posite window-sash. If there is not an exact neutralization, 
it can be seen at once which is the stronger. If the combina¬ 
tion causes the sash to move in the opposite direction, you 
know that the concave lens you have taken from your set of 
test-lenses is not strong enough to exactly neutralize the 
convex, and you take another and a stronger one, until you do 
find one that is just strong enough to neutralize the convex 
lens and the combination acts as a piece of plane glass. If, on 
the other hand, the combination causes the sash, when looked 
at through the lens, to move in the same direction, you know 
that the concave lens more than neutralizes the convex lens 
you are testing, and then you must take another and a weaker 
one, until you find one that is just weak enough (or strong 
enough) to exactly neutralize the lens that is being tested, 
when the combination will act as a piece of plane glass, and 
there will be no movement of the window-sash with movement 
of the combined lenses. 

To illustrate this subject of neutralization, when you take 
up a new lens you see at once that it is convex, and you judge 
by the rapidity of the movement of the window-sash that it is 
about + 3 D. You take up a — 3D. lens from your test-case, 
and, placing the two lenses together, and moving them while 
looking at the window-sash, you expect to find perfect neu¬ 
tralization, instead of which you find a slight movement 
against the lenses. You know from this that the convex lens 
is not yet fully neutralized, and as the movement is but slight, 
it becomes evident that the difference cannot be very great. 
You then try a — 4 D. lens, and now the movement is with 
the lenses, and you know that the convex lens is more than 
neutralized. Having thus found that — 3 D. is not strong 
enough, and that — 4 D. is too strong, you then try — 3.50 D., 
and this time you find that there is not the slightest movement 
when looking through the lenses in motion, and therefore you 


LENSES. 


89 


know that there is a complete and perfect neutralization; and 
as you know that the strength of the concave lens is — 3.50, it 
follows, as a consequence, that the strength of the convex lens 
must be + 3.50. 

LENS-MEASURE. 

The illustration, which is full size, gives a very good idea 
•of this useful little instrument, which is to be employed in 
accordance with the following 

DIRECTIONS. 

There iare three points (seen at top of the instrument), the 
two outer ones being fixed and the central one movable. The 
lens to be measured is pressed firmly upon these three points, 
thus causing a sinking of the central one, which acts on the 
index finger, rotating it until it points to a certain number on 



the scale, which will indicate the refraction in dioptres of this 
surface of the lens. If the index points to the left of 0 , the 
lens is convex; if to the right of 0, it is concave. 

Then turn the lens and measure the other surface. If 
both surfaces are convex, or both concave, the numbers are 


90 


LENSES. 


added together to determine the refractive power of the lens. 
For example, if one surface is + .75 D. and the other surface 
+ 1.25 D., the lens is a + 2. D. If one surface is — 1.25 D.. 
and the other surface — 1.50 D., the lens is a — 2.75 D. If 
one surface is convex and the other concave (as in the case of 
a periscopic lens), one is to be subtracted from the other; for 
example, if one surface is + 2.25 and the other surface — 1.25. 
D., the lens is a periscopic convex of 1. D. 

If the lens is rotated and the index points to the same 
number in all its meridians, the lens is proven to be spherical. 
If, however, there is a variation in the pointing of the index,, 
it cannot be a spherical lens, and it is presumed to be a cylin¬ 
der. In order to determine this we rotate the lens until the 
pointer stops at 0, which, being the meridian of no refraction,, 
is the axis of the cylinder; then rotate the lens at right angles 
or to the place where the pointer swings farthest from 0, when 
the strength of the cylinder can be read off. 

In compound lenses, each surface can be measured as 
above, and the determination of spherical and cylindrical 


,\ 







LENSES. 


91 


quickly made. When a plane lens is measured the index 
points to 0 . The inside large figures are dioptric numbers, 
and the outside small figures inch numbers. 

PRISM MEASURE.-TO CENTER LENSES. 

The lens to be centered is placed upon the front part of 
the bed-plate A, resting upon the lower points; the upper 
plate B is then pressed down until the points of the index 
finger touch the lens, and if the lens is of the same thickness 
at each point (that is, at D and D'), the index will point to 0 
on the scale, and the middle point ( N ) will be exactly over the 
center of the lens. (See cut on page 90.) 

TO MEASURE PRISMS. 

In order to determine the strength of a prism, the lens to 
be measured is placed in the same position as above, when the 



difference in the thickness of the lens at the points D and D' 
will cause the index finger to move along the degree circle, 
where the strength of the prism can be quickly read off. 







<92 


LENSES. 


TESTING A LENS FOR FLAWS. 

Noticeable cracks, bubbles, specks, or other flaws in a lens 
are usually discoverable on a casual examination, and without 
the necessity of making use of any special 'method of examina¬ 
tion; but they can be the more easily detected by viewing the 
lens with a bright light shining upon it from one side, and 
especially if the lens be held before a dark background, as in a 
room that is lighted by a single lamp or gas-burner. Reflected 
light from a window or lamp may be made use of to discover 
irregularities of the surface, by holding the lens in such posi¬ 
tions that there may be obtained, in turn, a reflection from 
every portion of the surface of the lens to be examined. In 
this manner of examination, any interference or marring of the 
perfect uniform reflection that ought to be obtained can be 
readily detected, and the lens set aside or accepted, as the case 
may be. 

Differences in the refractive power of certain portions of 
the same lens can be detected while making the attempt to 
neutralize the lens, by noticing whether there is imperfect 
neutralization in any portion of the lens. You expect to find 
neutralization equally perfect in all portions of the lens, and 
any imperfection in this respect in any portion of the lens can 
thus be discovered. 

In this connection it should be remembered that the 
double lenses of the test-cases do not always allow accurate 
neutralization, and this imperfect neutralization is the more 
noticeable with the stronger numbers, with which it is possible 
to neutralize only the center of the lens. Plano-convex or 
concave lenses are not so open to this objection, but can be 
more accurately neutralized than double lenses. 


CHAPTER V. 


NUMBERING OF LENSES. 


The handling and prescribing of lenses being the opti¬ 
cian’s chief work, it is just as necessary for him to have a clear 
understanding of the manner of their construction and meas¬ 
urement as it is of their proper adjustment. Unfortunately, 
just now is a transition period in their nomenclature, render¬ 
ing this subject of the numbering of lenses a matter of more 
than ordinary difficulty, for it requires a knowledge of both 
systems of numbering lenses to be able to intelligently under¬ 
stand the current works on the subject of optics. 

About thirty years ago the present system of measurement 
of lenses in inches was practically introduced. Previous to that 
time, and before the principles which should govern the ad¬ 
justment of spectacles had been investigated with scientific 
accuracy, and when people expected opticians and jewelers to 
prescribe spectacles as well as to make and sell them, there was 
no uniform system of numbering lenses, but those in common 
use were distinguished by arbitrary names or numbers. A 
manufacturer might make twenty grades of lenses, of no 
definite and fixed relation one to the other, and he would 
number them one to twenty as a matter of convenience. 
Another manufacturer might make only twelve grades of 
lenses, and number them from one to twelve, although they 
might embrace the same range as the one to twenty lenses. 
Every manufacturer would have his own system of numbering, 
and hence these numbers really meant nothing and conveyed 
nothing definite to the optician’s mind, because the number 
ten of one manufacturer might be the equivalent of number 
eight of another, or the number twelve of a third manufac¬ 
turer. Sometimes the glasses, especially in the lower powers 
of convex lenses, were called by arbitrary names expressive of 
their supposed or fancied properties, as “preservers,” “clear- 
ers,” and such like rubbish. All this was a source of endless 
confusion, and to overcome this muddled state of affairs, it 
gradually came to be understood that the number of the lens 
should indicate its focal length in inches. 

93 





94 


NUMBERING OF LENSES. 


This period of definite inch nomenclature commenced 
about i860, and people began then to think of their spectacles 
in inches. Under this system, when a man would come to the 
optician and say, “I have been wearing number ten,” the 
optician would understand that glasses of ten inches focal 
length were meant, and not some indefinite number ten of 
some manufacturer of the pre-scientific period. 

But although this was a great improvement over the old 
system of numbering, then a new source of error and confu¬ 
sion arose from the fact that the inches of different countries 
were not exact equivalents; the Parisian inch is the equivalent 
of 27.07 millimeters; the English inch of 25.30 millimeters; the 
Austrian inch of 26.34 millimeters, and the Prussian inch of 
26.15 millimeters. 

Now the refracting power of a lens depends also on the 
index of refraction of the glass, varying with the kind of 
which it is composed. The index of refraction of the glass of 
which lenses are constructed varies all the way from 1.526 to 
1.534, and hence there are sources of error in all calculations; 
for even though the country is known where the lenses are 
made, and, of course, presumably on the standard of that 
country, the exact refracting power can never be told unless 
the index of refraction of the glass is known as well. In order 
to simplify the latter, a common index of refraction of 1.5 was 
accepted; but even with that wrong basis, only part of the 
trouble was removed, so that, as a compromise, it became 
generally accepted that the number of a lens indicated both the 
focal distance and the refracting power. Thus a lens num¬ 
bered nine had a focal distance of nine inches and a refracting 
power of one-ninth. But it was really known all the time that 
it had not, and it in no way made an intelligent person feel 
that he had solved a practical matter by trying to deceive him¬ 
self with what he knew was not right. In other words, a 
sensible system of rotation would indicate either the power of 
refraction or the focal distance of a lens. This old system did 
neither, and by making the unit too strong necessitated the 
constant use of fractions in all calculations. Practically we 
have much more to do with the refracting power of a lens than 
with its local distance. The refracting power is always the 


NUMBERING OF LENSES. 


95 


inverse of the focal distance. The numbers of the old system 
give the focal distance of the lens in inches, the unit being a 
lens of one inch with the refracting power of y. There is 
seldom any need of this lens in practice, and it is not put into 
the ordinary trial-cases. 

According to the inch system of numbering spectacle 
lenses, a glass of one inch focus is taken as the standard; this 
unit being the strongest lens, all other lenses are weaker and 
must necessarily be expressed in fractions. There might be 
stronger lenses than this unit—as, for instance, a lens whose 
focal distance was only one-half inch, and whose refracting 
power would be double that of the unit—but such strong 
lenses are never needed or used for spectacle purposes, and 
hence we regard the unit or one-inch lens as the strongest of 
the series. A lens having a focal distance of two inches—that 
is, twice the length of the focal distance of the unit lens— 
would possess one-half its strength, and would be expressed by 
the fraction J. A lens having a focal distance of ten inches— 
that is, ten times the length of the focal distance of the unit 
lens—would possess only one-tenth its refracting power, and 
would be expressed by the fraction T V- A lens having a focal 
distance of seventy-two inches—that is, seventy-two times the 
length of the focal distance of the unit lens—would possess 
only one-seventy-second of its refracting power, and would be 
expressed by the fraction . Thus it will be seen that all the 
various lenses weaker in proportion are represented by corre¬ 
sponding fractions, and the denominator of the fraction always 
expresses the focal distance and refracting power. 

This is the system, then, that has been in common use 
until of late years, and is still adhered to by the older opticians 
of the present day; and although a fairly satisfactory system, 
it is open to many objections, a few of which I will mention. 

The chief difficulty occurred when it was desired to com¬ 
bine lenses together, as the addition had to be made entirely 
in fractions. When a seventy-two-inch lens and a twenty-inch 
lens were to be combined, we had to deal with their refracting 
powers, and not their focal distances; in the case just men¬ 
tioned, the focal distances would be seventy-two inches and 
twenty inches, while the refracting powers would be and 


96 


NUMBERING OF LENSES. 


and as it is the latter only that are to be considered, it is evi¬ 
dent that the addition must be made in fractions. In this case 
we have + yV as the problem; and while every schoolboy 
knows how it can be done, it is, more or less, a tedious process, 
and takes a little time, involves some figuring, and scarcely can 
be done in the head. We reduce them to a common denomina¬ 
tor, and we have and Ter = rlfa- Then r ihs + tHf 

— 144 0 ') or about T V%, which we call y^. Who will not say 
that the necessity that constantly arises in the every-day ex¬ 
perience of all opticians for the frequent addition and subtrac¬ 
tion of fractions, like the above, in the combination of ordi¬ 
nary lenses, is a powerful objection to the inch system, as it is 
a most troublesome, difficult and tedious process, and one that 
is fraught with many liabilities of error and mistake? 

Another objection that may be urged against the inch 
system is that the intervals between the lenses are not regular 
nor uniform; for instance, the difference between an eight-inch 
and a nine-inch lens—that is, between a -J and a y lens—is 
much greater than between a twelve-inch lens and a thirteen- 
inch lens—that is, between a T V and a T y. The interval 
between any two lenses of the inch system is never the same as 
between two other lenses of the same system, the difference 
between a T y and a y^ lens is not the same as between a T V and 
a and, as can be readily seen, the lower down in the scale 
you go, and the stronger the numbers become, the greater the 
difference between the adjoining numbers. This lack of uni¬ 
formity in the intervals between the lenses of the inch system 
constitutes an objection to that system which is scarcely less 
formidable than the first-mentioned one of the difficulty of 
combining fractions. 

The last objection, and which I will merely mention, is 
that the standard or unit of the inch system (one inch) is so 
strong that it is seldom, if ever, used. 

THE DIOPTRIC SYSTEM. 

To overcome and obviate the difficulties of and objections 
to the inch system, many plans and systems have been offered 
by different oculists and introduced at their various conven¬ 
tions and meetings. This resulted in the proposal, at the 


NUMBERING OF LENSES. 


97 


International Congress of Ophthalmology in 1867, of a new 
system of numbering all lenses according to their refractive 
power; this was followed by the decision of the Ophthalmo- 
logical Congress, in 1872, to adopt a metrical scale of measure¬ 
ment, which new or metric system was finally adopted by the 
Ophthalmological Society, which convened at Heidelburg in 

1875- 

This system selects for its unit a lens with a focal distance 
of one meter (instead of one inch, as in the old system), and 
which is called a dioptre , it being also written dioptry and diop¬ 
tric, and is represented by the abbreviation D. In this system, 
then, a weak number instead of a strong one (as in the inch 
system) is used for the unit, and, as the majority of the lenses 
used are stronger than this, their refracting power can be rep¬ 
resented in whole numbers. A lens of twice the strength is a 
two-dioptre (or 2 D.), and it has a focal distance of one-half 
meter. A lens of four times the strength is a four-dioptre (or 
4 D.), and has a focal distance of one-quarter of a meter. It 
will be seen that in all these lenses the focal distance is always 
some fraction of a meter, while the number of the lens 
expresses its refracting power, and not its focal length as in the 
old system. This gives us a series of lenses with an equidis¬ 
tant interval of one dioptre, and the numbers 1 D. to 20 D. 
indicate the uniformly increasing power of the glasses. 

Unfortunately, for the practical uses of the optician there 
is need of lenses weaker than one dioptre and at intervals 
between the dioptres, so that this system does not, after all, 
remove the need of fractions, but it substitutes for the vulgar 
fractions the so-called decimal fractions, which do not add 
anything to the difficulty of combining such lenses. This fur¬ 
nished us with three intermediate lenses between the dioptres 
and the same number of lenses weaker than a dioptre, viz,, 
0.25 D., 0.50 D. and 0.75 D., or one-quarter of a dioptre, one- 
half a dioptre and three-quarters of a dioptre as expressive of 
their refracting power, while their focal distances would be re¬ 
spectively four meters, two meters and one and one-third 
meters. 

Of late years there has been a tendency to make a still 
finer subdivision of lenses by the introduction of a .12J D. 


9 8 


NUMBERING OF LENSES. 


lens, which would furnish us four more lenses between the 
dioptres, and the same number of additional lenses weaker 
than the dioptre, as follows: . 12 J D., .25 D., . 37 I D., .50 D., 
. 62 J D., .75 D., . 87 I D., 1 D., 1.124 D., 1.25 D., 1.374 D., 
1.50 D., 1.624 D., 1.75 D., 1.874 D-, 2 D., and so on. The 
value of these fractions of 4 is so trifling that they can be dis¬ 
carded without detriment, and the series would then read, . 12 , 
•25, *37> -5°, - 62 , . 75 , .87 and 1 D. 

This gives the optician a series of lenses that may fairly 
be called complete, and with an equidistant interval between 
each one. 

The two chief objections urged against the inch system— 
that is, the difficulty of combining the vulgar fractions of 
which they are composed, and the irregular intervals between 
the lenses—are entirely removed in the dioptric system, as the 
addition or subtraction of dioptres is easy and simple to the 
extreme, while the interpolation of the decimal fractions adds 
nothing to the difficulty of such addition or subtraction, it 
being done as easily as the whole dioptres; in fact, it is the 
same as the addition or subtraction of dollars and cents. 

METRIC AND INCH SYSTEMS COMPARED. 


The following table explains itself, and should not only 
be carefully studied, but should be kept convenient for ready 
reference: 


Dioptric 

System. 

English inch 

1 meter = 
about 40 inches . 

French inch 

1 meter = 
about 36 inches. 

Dioptric 

System. 

English inch 

1 meter = 
about 4 0 inches. 

French inch 

1 meter — 
about 36 inches. 

0.12 

320 

288 

5. 

8 

7 

0.25 

160 

144 

5.50 

7 

6% 

0.37 

108 

99 

6 . 

6 % 

6 

0.50 

80 

72 

6.50 

6 

5 % • 

0.62 

64 

58 

7. 

5 % 

5 

0.75 

53 

48 

7.50 

5g 

4 4-5 

0.87 

46 

41 

8 . 

5 

4 % 

1 . 

40 

36 

8.50 

4 % 

4% 

1.25 

32 

29 

9. 

4 % 

4 

1 50 

26 

24 

9.50 

4 1-5 

3% 

1.75 

22 

20 

10 . 

4 

3 3-5 

2 . 

20 

18 

11 . 

3% 

3% 

2.25 

18 

16 

12 . 

3 % 

3 

2.50 

16 

14 

13. 

3 

2 10-13 

2.75 

14 

13 

14. 

2 % 

2 4-7 

3. 

13 

12 

15. 

2% 

2 2-5 

3.25 

12 

11 

16. 

•M 

2 % 

3.50 

11 

10 

17. 

2% 

234 

3.75 

10 % 

9% 

18. 

2% 

2 

4. 

10 

9 

19. 

2 1-9 

117-19 

4.50 

9 

8 

20 . 

2 

1 4-5 
















NUMBERING OF LENSES. 


99 


The first column gives the number of the lens ground 
according to the new dioptric or metric system, and the second 
and third columns its equivalent in English and French inches 
respectively. The table shows the focal distance of the lenses 
in general use, and affords an understanding of the relative 
value of the old and new systems. The meter is taken as the 
standard, and is considered equal to forty English inches or 
thirty-six French inches; these are not the exact figures, the 
equivalent in English inches being a fraction over thirty-nine 
inches, and in French inches a fraction over thirty-six inches, 
but the whole numbers of forty and thirty-six are sufficiently 
accurate for all practical purposes, and are universally used In 
making the comparisons between the two systems. The 
French inch may be left out of the question, as the English 
inch is the one we use, and hence all the calculations are made 
on the basis of a meter equaling forty inches. 

The test-cases used by oculists and opticians were, until 
recently, composed of lenses ground according to the French 
inch system; and it can be easily seen how an error and much 
inconvenience might result, with corresponding discomfort to 
the patient’s eyes, if a prescription based upon the French inch 
test-cases was filled by an optician whose lenses were ground 
according to the English inch system, and who, therefore, 
failed to furnish the glasses intended by the prescriber; and 
the difference would be all the more marked if the lenses pre¬ 
scribed were among the stronger numbers. 

In converting inches into dioptres, or dioptres into inches, 
we make use of the following simple rule: To find the focal 
distance of a lens in inches, we divide forty by the number of 
the dioptres; and to find the number of the lens in dioptres, 
we divide forty by the number of inches. 

According to the above method of calculation, the focal 
distance of a lens is the inverse of its refractive power. Take, 
for example, a lens of 4 D.; if we wish to find its equivalent in 
the inch system we divide it into forty, equaling ten inches; 
or as the focal distance of a lens is the inverse of its refractive 
power, then with a lens of 4 D. we have the following: 


1 m. 


= 25 cm. = 10 inches. 


4 



IOO 


NUMBERING OF LENSES. 


This means that one meter, or its equivalent one hundred cen¬ 
timeters, divided by four equals twenty-five centimeters, which 
in turn equals ten inches. 

In the same way we can find the number of the dioptre, 
which is the inverse of the focal distance. Take, for example,, 
a lens of eight inches focal length, which is equal to twenty 
cm., and we then have the following: 

1 m. 100 cm. 

20 20 = 5 ^)- 

The simple rule to be remembered in making any and all 
calculations in the conversion of inches into dioptres or diop¬ 
tres into inches, is in the one case to simply divide forty by 
the refracting power in inches (which gives the number of 
dioptres), and in the other case to divide forty by the number 
of dioptres (which gives the number of inches). This simple 
rule of calculation is in constant use, and if the optician is at 
all apt with figures, the calculation can usually be made men¬ 
tally without any recourse to figuring with paper and pencil. 
After some experience with the two systems, and the constant 
conversion of one into the other, which their use necessitates, 
the optician soon has their equivalent values fixed in his mind, 
so that when he picks up a lens marked in inches, its value in 
dioptres at once occurs to his mind, and that without any 
mental effort on his part. 

One of the advantages that might be mentioned of using 
the metrical system, is that it prevents your customer from 
knowing or learning the inch number of the glasses he is or 
should be wearing. Some persons are unkind and dishonest 
enough, after the optician has spent a great deal of time in 
testing their eyes, to ascertain from him the proper number of 
the glasses they need, and then, with some trivial excuse or 
with the remark that they will call again, they go around the 
corner to some street peddler of spectacles and ask for the 
number the optician found to be suitable for their eyes. This 
can be prevented by the constant use of the dioptric system of 
numbering; or, what is still better, in the great majority of 
cases, is not to let the customer know the number of the glass 
that is given him or that suits him. If he learns this he is 
independent, and in case he breaks or loses his glasses he can 


NUMBERING OF LENSES. 


IOI 


go to any peddler or general merchandise store and obtain the 
proper number of glasses; otherwise he must return to the 
optician who furnished his glasses when he needs a new pair, 
and thus he can be kept somewhat dependent upon the opti¬ 
cian, with a bond more or less firm, as the capabilities of the 
optician will warrant. Or, if the customer makes the request 
to know the number of the lenses that he needs, the optician 
can truthfully reply that his lenses are not ground according to 
the common inch system, but according to the new French 
metrical system, without, of course, any mention being made 
to the customer that even though they are numbered on the 
metrical system, they might have an equivalent in the more 
common inch system. This sometimes satisfies the customer, 
and no more questions are asked. If, however, he is not satis¬ 
fied with this evasive reply, and the optician feels he cannot 
refuse to give him the number, it is given according to 
the metrical system with the dioptric number—it may be 3 D., 
it may be 2 D., it may be 1 D.—and he is simply told it is 3 , 2 
or 1 , as the case may be; and if he goes off with the intention 
of purchasing this number elsewhere, and asks for it from an 
optician who is not thoroughly posted, it would be next to im¬ 
possible for him to obtain the proper glasses, and he would 
begin to think that the first optician probably understood best 
how to fit his eyes after all. 


CHAPTER VI. 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 


The eye is certainly the most useful, as it is the most 
wonderful, of all our organs of special sense. The organs of 
touch, taste and smell, in order to perform their functions, 
must be placed in actual contact with the foreign substances 
which excite their activity; but the sense of sight is not so 
limited, but is equally sensitive to the impressions of light, 
whether it comes from an object close at hand, or from the 
immeasurable distances of the fixed stars. 

The eye is in more direct connection with the brain and 
mind than is any other organ, and thus it often expresses the 
strongest passions and the most tumultuous emotions, as well 
as the gentlest thoughts and most delicate sentiments. Much 
of this external intelligence that dwells in the eyes is marred in 
persons who squint or who are near-sighted. How often are 
we influenced in our judgment of the character of others,whom 
we meet for the first time, by the expression of their eyes. 

The cavity of the eye-ball is like a room with but one 
window, where all the light which enters must come from the 
front, and necessarily strikes the back wall of the apartment. 
The construction of the eye-ball, in its general arrangement as 
an organ of vision, is very much like an optical instrument, and 
as such is subject to the same physical laws as govern any 
other optical instrument. Images of external objects are 
formed in the eye exactly as they are formed in a photogra¬ 
pher’s camera, where they fall upon a chemically sensitive 
plate and are made permanent by the chemical changes in¬ 
duced by light. In the eye they fall upon the nervously 
sensitive retina, and the impression they make is immediately 

102 




THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. I03 

conveyed to the brain by the fibres of the optic nerve, so that it 
is really not the eye that sees, but the brain, as it is only after 
the brain takes cognizance of the image that is formed in the 
eye, that the visual act is complete. 

The convex lens of the camera, which can be screwed in 
and out to receive clear images of objects at different distances, 
is represented in the eye by the crystalline lens, which has the 
faculty of changing its convexity, and thus accommodating 
the eve for far and near distances. The blackened inner sur¬ 
face of the camera is represented by the choroid, which lines 
the whole inner surface of the sclerotic with a dark pigment, 
and thus prevents reflections within the eye and absorbs the 
excess of light which has passed through the substance of the 
retina. 

The conditions necessary for clear and satisfactory vision 
are as follows: 

1 st. A well defined image must be formed on the retina at 
the yellow spot. 

2 d. The impression there received must be conveyed 
quickly and directly to the brain. 

The optician is more particularly concerned in the first of 
these conditions, as it is his business to so correct existing op¬ 
tical defects as to make it possible for a distinct image to be 
formed upon the retina. 

But if, after a perfect image is formed, the conducting 
power of the nerve is so much impaired as to be incapable of 
conveying the impression of the image to the brain, then the 
case passes beyond the province of the optician, and requires 
treatment at the hands of the oculist. 

If either the conducting function of the optic nerve or the 
perceptive function of the retina should be abolished, there 
would be no vision at all; but when the retina and optic nerve 
are healthy, the quality of vision depends entirely upon the 
transparency of the refracting media and upon the perfection 
of the optical images which they form. Vision may therefore 
be imperfect either because the refracting media of the eye 
have lost their translucency, or because the images cast upon 
the retina are blurred by some optical defect. 


104 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

THE REFRACTING MEDIA. 

The refracting media of the eye are the cornea, the aque¬ 
ous humor, the crystalline lens, and the vitreous humor. 

The cornea is capable in a twofold manner of refracting 
and converging the rays of light that fall upon and traverse it; 
it affects them first by its density, and in the second place by 
its convexity. After the ray of light has passed through the 
cornea, it next traverses the aqueous humor, which humor 
affects the ray but very little, its chief use being to maintain 
the proper convexity of the cornea, and at the same time to 
furnish a medium in which the movements of the iris can take 
place. 

In regard to the vitreous it may be said that its principal 
use appears to be to give the proper distention to the globe of 
the eye-ball, and also to keep the surface of the retina at the 
proper focal distance from the lens. 

The crystalline lens is the most important refractory me¬ 
dium of the eye; it acts, by virtue of its double convex form, as 
a converging lens, bringing to a focus the luminous rays that 
pass through it. 

The function of the crystalline lens is to produce distinct 
perception of form and outline. If the eye consisted merely of 
a sensitive retina covered by a transparent membrane, the im¬ 
pressions of light would be received, but would afford no idea 
of form or outline, producing merely the sensation of confused 

light, amounting sim- 
' ply to the perception of 
light from darkness. 
% Such a condition is 
s illustrated by the ac- 
companying diagram. 

The arrow represents a luminous body, while the vertical 
line at the right represents the retina. The rays which diverge 
from the point of the arrow reach every part of the retina, and 
in like manner the rays which diverge from the butt of the 
arrow reach every part of the retina; consequently the differ¬ 
ent points of the retina each receive rays coming from both the 
point and butt of the arrow. There can therefore be no dis- 


2>\4U?rani _A 










THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. IO5 

tinction by the retina between the point and the butt of the 
arrow and no 'definite perception of its figure. 

But if now there is supplied between the arrow and the 
retina a double convex lens of the proper focus, the effect 
will be entirely differ- j>iagrr*nt b 

ent, as shown in the 
accompanying dia- 
gram. 

In this case all 
the rays emanating from the point of the arrow will be con¬ 
centrated at one certain spot, and all the rays emanating from 
the butt of the arrow at another certain spot. Hence the 
retina receives the impression of the point of the arrow sepa¬ 
rate and distinct from that of its butt; and all parts of the 
arrow will be separately and distinctly perceived. 

From the foregoing figures it is easily seen that distinct 
perception of the form of an external object is only possible 
when all the rays of the light emanating from each and every 
point of the object are accurately focused on the retina by the 
crystalline lens of the eye. In order to accomplish this satis¬ 
factorily, the density of the lens, the curvature of its surfaces, 
and its distance from the retina, must all be in proper propor¬ 
tion to each other. 

If the lens is too convex, or if it is too far from the retina, 
the rays would meet in focus before reaching the retina, and 

would cross each other and 
fall upon the retina in dif¬ 
fusion circles, as is illus¬ 
trated in the accompanying 
diagram. 

The image from a case of excessive refraction, as shown 
above, would not be clear and distinct, but would be diffused 
and indistinct, because the rays of light, instead of being con¬ 
centrated to a definite point, are dispersed more or less over 
the surface of the retina. 

If, on the other hand, the lens is too flat, or if it is too 
near the retina, the image again would be confused and indis¬ 
tinct, as illustrated in the following diagram: 

















106 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION, 

The image from a case 
of deficient refraction, as 
shown here, would not 
be clear and distinct, but 
would be diffused and in¬ 
distinct, because the rays of light are dispersed more or less 
over the surface of the retina, instead of being concentrated to 
a definite point. In both of the above cases the rays of light 
strike the retina in diffusion circles without producing any well 
defined image; in the first case, because they have actually 
converged and crossed each other, and in the second place, 
because they have only approximated but never converged to 
a focus. 

As has been stated, the rays of light, after being con¬ 
verged by the crystalline lens, form their image upon the 
retina, which, consequently, is the most essential part of the 
organ of vision, as it is the only one of its tissues directly sen¬ 
sitive to light. The retina is a delicate, transparent membrane, 
composed largely of nervous elements, and lining the whole 
interior of the cavity of the eye-ball. The retina seems to be 
a continuation of the optic nerve, which enters the eye-ball by 
piercing the outer coats and spreads out to form this mem¬ 
brane. It has been found by microscopists to consist of a 
number of different layers, which together form this mem¬ 
brane, the whole being connected with the extremities of the 
optic nerve fibres. 

On account of the delicate nervous structure of the retina, 
it is well adapted to receive the impressions of the rays of light, 
and by means of its intimate connection with the optic nerve, 
to convey such impressions to the brain. 

All parts of the retina are not equally sensitive to light; 
there is one portion called the yellow spot —from its color—that 
is more sensitive than any other part. In fact, it is the only 
spot on the whole retina that affords clear and perfect vision, 
while vision becomes gradually more and more imperfect, as 
the image is impressed upon the retina farther and farther 
from this yellow spot. Consequently it becomes necessary in 
reading to move the eyes backward and forward along the 
lines of the print, so as to bring each word of every line, and 


J)xcL<jretni D 







THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. IO/ 

each letter of every word, in the direct line of vision, in order 
that its image may fall upon this sensitive yellow spot. 

So in distant vision, the eyes must be in continuous, 
though unconscious, motion, in order that the different objects 
around us may be placed in such a position, in reference to the 
eye, that their images may again fall upon this sensitive yellow 
spot. An active, sprightly person moves his eyes quickly from 
object to object, so as to see everything clearly by bringing 
everything into a direct line of vision with this yellow spot; 
while dull, phlegmatic people are satisfied with a general view 
of things as their images may happen to fall upon any part of 
the retina, without taking the trouble to move their eyes and 
adjust their accommodation, so that the same images may be 
sharply focused upon the yellow spot. 

THE BLIND SPOT. 

In contrast with and very near to this sensitive yellow 
spot, there is a small spot that is insensible to the rays of light, 
which means that it is a blind spot. At first thought it seems 
somewhat strange that there should be a blind spot in every 
man’s eye, and also that this spot of least vision, or blind spot, 
should be so near to the spot of best vision, or yellow spot. 
But this is the case, and a still more curious fact is, that this 
blind spot is just at the entrance of the optic nerve, where it 
would naturally be thought that vision ought to be the most 
acute. The explanation of this is that the nerve fibres here 
belong to the conducting layer of the retina, while the per¬ 
cipient layer is wanting at this point. 

It would seem reasonable to suppose that, if there is a 
blind spot in every one’s eye, there ought to be a correspond¬ 
ing dark spot in the field of vision. Such is not the case, for 
the following reasons: When the eye is directed toward any 
object, to see it, the image falls upon the yellow spot, which is 
in the visual axis of the eye, while the blind spot is situated a 
little to the inner side of this point. Consequently, the image 
of an object, which is directly examined in the normal line of 
vision, cannot fall upon this blind spot. 

When both eyes are open, an object may be so placed 
that its image falls upon the blind spot of one eye, in which 


to8 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

case, however, it will necessarily fall upon the yellow spot of 
the other eye, and the object be distinctly seen. It is impossi¬ 
ble for an object to be placed in such a position that its image 
could fall upon the blind spot of both eyes at the same time. 

If, however, only one eye is used, there is always a small 
portion of the field of vision that is imperceptible. This de¬ 
ficiency is not noticeable, because it is located in a part of the 
field of vision to which our attention is scarcely directed, and 
where the perception of various objects is so imperfect that the 
momentary absence of one of them is not regarded. That this 
blind spot does exist can be readily made apparent, and any 
one can observe it for himself, by using for the test a single 
strongly defined object, like a white spot on a black ground, 
or a black spot on a white ground, the presence or absence of 
which may be quickly noticed. 

* © 

The left eye is to be closed and the right eye to be directed 
Steadily at the cross on the left-hand side of the illustration; 
the round spot will also be visible, though less distinct than 
the cross, because it is not in the line of direct vision. Let the 
page be held vertically before the eyes, and at a convenient 
distance for seeing both objects in the manner just mentioned. 
If it now be moved slowly backward and forward, a point will 
be found at a certain distance, about ten or twelve inches, 
where the circular spot disappears from view, because its image 
has fallen upon the optic nerve entrance or blind spot, which 
is insensible to the rays of light, to reappear again if the paper 
be moved nearer or farther. It may also be made to reappear, 
even at the same distance, by inclining the page laterally, to 
the right or left, since this would bring the circular spot either 
above or below the level of the blind spot. The phenomenon 
of the blind spot is well illustrated in the following figure. 




THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. IO 9 


THE FIELD OF VISION. 

As a man looks out upon a landscape, there is in front of 
each eye a circular space within which objects are distinctly 
perceptible, while beyond its borders vision fades away to 
nothingness. This circular space is called ‘‘the field of vision.” 
In man it has quite a large limit; for instance, when the eye is 
directed straight forward, the light from a brilliant object may 
be perceived when the object itself is placed away around to 
one side. In many of the lower animals, where the eyes are 
more prominent than in man, the field of vision is very much 
enlarged. In birds and fishes it is still further enlarged by the 
lateral position of the two eyes. The ostrich, when its head is 
directed forward, can easily see objects placed a few yards 
behind its back; the field of vision for such an animal is con¬ 
sequently a complete sphere, objects being perceptible in 
every direction. 

In man the field is limited, and objects placed laterally at 
the external borders of the field must be very brilliant to at¬ 
tract attention. Within this field there is only one point where 
objects can be seen with perfect distinctness; it is in the center 
of the field, and its prolongation forward from the pupil is 
called the “line of direct vision.” Objects met with upon this 
line can be distinctly seen; all other objects, situated upon 
either side, above or below it, are seen more or less imper¬ 
fectly. If one places himself in front of a fence composed of 
vertical stakes, he can see those placed directly in front of the 
eye with perfect distinctness, while those on either side begin 
to appear uncertain and confused. On looking at the center 
of a printed page in the line of direct vision, we can see the 
distinct outlines of the letters; while at successive distances 
from this point, as the eye remains fixed, we distinguish at 
first only the separate letters with confused outlines, then only 
the words, and lastly, only the lines and spaces. 

This limitation of distinct sight to the line of direct vision 
is practically compensated for by the great mobility of the eye¬ 
ball, which rapidly turns in all directions, thus shifting the 
line of direct vision, and examining in turn every part of the 
field of vision attainable by the eye. In reading this article 


IIO THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

the eye follows the lines from left to right, seeing each letter 
and word distinctly in succession. At the end of each line it 
returns, suddenly, to the commencement of the next, repeating 
the same movement from the top to the bottom of the column. 

The cause of the indistinctness of the images seen outside 
the line of direct vision is a twofold one: first, because the 
rays of light, on account of the oblique manner in which they 
enter the eye, are more rapidly converged and consequently 
are not accurately focused upon the retina; in the second 
place, a perfect image is impossible, on account of the dimin¬ 
ished acuteness of the retinal sensibility at every part of the 
retina except at the yellow spot. 

In the formation of well-defined images on the retina to the 
end that perfect vision may result, there are three factors in¬ 
volved, as follows: Refraction , Accommodation, Convergence. 

REFRACTION. 

Distinct vision necessarily depends upon the rays of light 
which enter the eye, being brought to an accurate focus upon 
the retina. The refracting media of the eye are the cornea, 
the aqueous humor, the lens and the vitreous humor, which, 
taken collectively, may be regarded as forming a single lens, 
the focal length of which is precisely equal to the length of the 
axis of the eye-ball. The normal human eye may be defined 
as an optical apparatus of such form that parallel rays of 
light—that is, rays proceeding from a distance of twenty feet 
or more—are precisely focused upon the retina without any 
effort upon the part of the eye, thus imprinting upon this sen¬ 
sitive membrane a sharply defined image of all objects from 
which these rays emanate. This is the condition known as 
emmetropia, this word being derived from two Greek words 
signifying that it is in measure. 

When this normal condition is departed from in any di¬ 
rection, then we have the condition known as ametropia, signi¬ 
fying that it is out of measure. This departure from the 
normal condition may be in three different directions. 

In one case the eye-ball may be flattened from before 
backward, in which case the rays of light, instead of being 
focused upon the retina, do not come to a focus until they get 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. Ill 

behind this membrane; this state of affairs constituting the 
condition known as hypermetropia. 

In another case the eye-ball may be elongated from before 
backward, in which case the rays of light, instead of being 
focused upon the retina, come to a focus before they reach this 
membrane, thus constituting the condition known as myopia. 

Either of these departures from the emmetropic condition 
may exist from a very small degree to a very great degree. 

This figure illustrates very graphically the extremes of 
hypermetropia and myopia as compared with emmetropia, 
having been taken from actual measurements of defective eyes: 



Diagram showing an emmetropic eye as compared with an extremely hyper¬ 
metropic and an extremely myopic eye. 


In another case the curvature of some one of the meridi¬ 
ans of the cornea of the eye may be flattened or elongated, 
thus causing the different rays to meet at different focuses, and 
constituting the condition known as astigmatism. 

In all three of the above mentioned cases the result is the 
formation of indistinct images upon the retina and an inevit¬ 
able impairment of vision. 

This action of the eye upon light is called its refraction , 
the above illustrations referring only to parallel rays of light, 
or those proceeding from a distance of twenty feet or more. 

If the eye was a rigid organ and had no inherent power of 
its own to act on the rays of light received within it, it is evi¬ 
dent that its refraction of light would always be the same: in 
emmetropia parallel rays would always be focused upon the 
retina; in hypermetropia the similar rays would always be 


II2 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

focused behind the retina (we say behind the retina because 
that would be their focal point if they could pass through the 
membranes of the eye) ; in myopia similar rays would meet in 
focus in front of the retina. 

Refraction (this definition is repeated because every op¬ 
tician should clearly understand what it means) is the action of 
the eye on light when the eye is passive or at rest; that is, when 
it is not called upon to exert any power of its own to assist in 
bringing the rays of light to a focus. And consequently when 
the refraction of the eye is referred to, there is meant to be 
expressed that the eye when in a state of rest is either emme¬ 
tropic, hypermetropic or myopic. 

In the two latter cases, as can be easily understood by a 
reference to the figure given above, the rays of light that fall 
upon the retina can not produce clear or well-defined images 
of objects, but vision is blurred and indistinct. In myopia the 
rays fall upon the retina after having come to a focus and then 
overcrossed; in hypermetropia the rays have not vet come to 
a focus, and fall upon the retina in this ununited condition. In 
both cases the retina receives only a patch of light (technically 
called a diffusion circle), instead of the defined image which is 
absolutely essential to satisfactory vision. 

HOW OPTICAL DEFECTS ARE CORRECTED. 

As has been shown in the chapter on lenses, the property 
of convex lenses is to render parallel rays of light convergent, 
and the property of concave lenses is to render parallel rays 
divergent. If now these properties of convex and concave 
lenses are made applicable to the correction of the defects of 
the eye, the optician can readily understand that a convex lens 
placed before a hypermetropic eye brings the rays of light to 
an earlier focus, and if the convex lens is of the proper strength 
to correspond to the degree of the hypermetropia, then the 
focus of the rays of light will be brought so far forward as to 
exactly correspond to the retina and will, therefore, be focused 
upon the retina. 

If on the other hand a concave lens be placed before a 
myopic eye, the rays of light are made divergent, and thus 
made to come to a later focus; and if the concave lens is of the 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. II3 

exact strength to correspond to the degree of the myopia, then 
the focus of the rays of light is thrown so far back as to corre¬ 
spond to the position of the retina, and the focus will, there¬ 
fore, fall upon the retina. If the above statements are care¬ 
fully considered, it can readily be seen how these optical 
defects can be corrected by the adjustment of the proper 
glasses. 

The degree of the myopia or hypermetropia is expressed 
by the strength of the lens required to correct it. A hyperme¬ 
tropia of one dioptric is one that requires a convex lens of one 
dioptric (+ i. D.) to correct it; a myopia of two dioptrics is 
one that requires a concave lens of two dioptrics (— 2. D.) to 
correct it. 

In addition to these defects there is a third one, which has 
already been mentioned as astigmatism, in which one of the 
meridians of the eye is normal while the meridian at right 
angles to it is either hypermetropic or myopic. Such an eye 
may be emmetropic for vertical lines, and hypermetropic or 
myopic for horizontal lines, or vice versa. In such an eye the 
rays of light entering through its defective meridian are fo¬ 
cused either before or behind the retina; while the rays entering 
through the emmetropic meridian are focused upon the retina; 
this confuses the sight and renders it more or less imperfect. 

The test for astigmatism is that in looking at a card of 
radiating lines they are not all seen with equal distinctness, 
some being much clearer than others, while in extreme degrees 
of the defect some of the lines may be almost or altogether 
invisible. 

It is well to know that the meridians of greatest and least 
curvature are always at right angles to each other. Astigma¬ 
tism diminishes the acuteness of vision more or less markedly, 
and sometimes quite curiously; this is shown in the case of a 
man who consulted an oculist for what he called “periodical 
obscuration of vision,” because he could see plainly the hands 
of the clock in his office at certain times of the day, while at 
other times he could scarcely see them at all. The oculist 
found that this peculiar vision was owing to astigmatism, that 
when the hands of the clock were in the meridian of good 
sight he could see them plainly; while when the hands had 


114 the eye optically; or, the physiology of vision. 

moved to the meridian of defective sight he could scarcely set 
them at all. 


THE ACCOMMODATION OF THE EYE. 

An optical instrument, composed of refracting lenses, can 
not be made to serve at the same time for near and far dis¬ 
tances. In a refracting telescope or spy-glass, if the instru¬ 
ment be directed toward any part of the landscape, objects at a 
certain distance only are distinctly seen; all other objects, sit¬ 
uated within or beyond this distance, are obscure or imper¬ 
ceptible. This is necessarily the case, since a lens or system 
of lenses can bring to a focus at one spot only those rays which 
strike its anterior surface with a certain degree of divergence. 
The formation of a visible image at the desired spot depends 
entirely upon the refracting power of the lenses being such 
that all the rays diverging from a particular point of the object 
shall be again brought to an exact focus at the plane where the 
image is to be perceived. If the object be placed at an indefi¬ 
nite distance near the horizon, or if it be one of the heavenly 
bodies, the rays emanating from any one point of such an 
object reach the telescope under so slight a degree of diverg¬ 
ence that they are practically parallel, and on being refracted 
they will be brought to a focus at a short distance behind the 
lens. 

But if the object be nearer, the rays emanating from it 
strike the lens under a higher degree of divergence. The same 
amount of refractive power in the instrument produces a less 
rapid convergence than in the former case, and the rays are 
consequently brought to a focus at a greater distance behind 
the lens. To provide for this difficulty the spy-glass is con¬ 
structed with a sliding tube, by which the distance of the eye¬ 
piece from the object glass may be changed at will. For the 
examination of remote objects the eye-piece is pushed forward 
so as to bring into view the image formed at a short distance 
behind the lens; for the examination of near objects it is 
drawn backward to receive the image placed farther to the 
rear. This is the accommodation of the spy-glass for vision 
at different distances. 



THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 


ACCOMMODATION. 

A similar necessity (for distinct vision at different dis¬ 
tances) exists in the optical apparatus of the eye. We have 
seen that the emmetropic eye in its passive condition (that is, 
when at rest and without making any effort) focused parallel 
rays of light exactly upon the retina. When, however, the 
rays proceed from objects nearer than twenty feet they are no 
longer parallel, but are then divergent, and consequently in 
this condition cannot be focused upon the retina unless there is 
some change in the refractive condition of the eye, because 
such rays require more converging than parallel rays in order 
to be focused at the same distance. The nearer an object 
approaches to the eyes, and consequently the more divergent 
the rays that proceed from it, the greater the amount of con¬ 
verging (and therefore the stronger the refracting lens) that 
will be required, provided it is desired to keep the focus at the 
same distance. 

The eye must possess some means of increasing its re¬ 
fractive power in order that these divergent rays of light may 
still be sharply focused upon the retina, or else we would all 
be deprived of the pleasure of clear vision of close objects; 
because, as we have already seen, the distinctness of the image 
formed in the eye of the observer depends upon the rays of 
light being brought to a perfect focus upon the retina. 

That the eye does possess such a power of variation of 
sight for different distances can be proven not only by reason¬ 
ing, but by direct experiment. This is shown in the case of the 
emmetropic eye, by which objects situated at various distances 
from the eye can within a certain range be seen with almost 
equal distinctness; as, for instance, such an eye can read the 
large letters on a sign a hundred feet or more away, the letters 
being clear and distinct, and the next second the same eye can 
read a page of smallest type held a few inches from the eyes, 
the letters being equally as distinct as in the case of the sign 
one hundred feet away. If the type is brought closer to the 
eyes, the individual becomes conscious of a sense of effort 
which increases as the type gets nearer, until presently the 
type gets so near that the divergence of the rays proceeding 


Il6 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

from it can no< longer be overcome, and then the letters be¬ 
come blurred; this shows that there must exist some provision 
by which the eye is enabled to adapt itself to vision at different 
distances, so that, whatever length the focal distance may be, 
the focal point may always fall exactly upon the retina. 

An illustration of this adaptability of the eye that has been 
frequently used, but that is as good as it is old, is to stretch in 
front of the eye, at a distance of seven or eight inches, a plain 
gauze veil or other woven fabric formed of fine threads with 
tolerably open meshes, so that objects beyond may be readily 
visible through its tissue. If this veil be held between the eye 
and a printed page (using only one eye) we can see at will 
either the threads of the veil, or the letters on the printed page 
through the interstices of the veil, but we cannot distinctly see 
both at the same time. When we see the threads of the veil 
sharply defined, the printed page is so blurred and indistinct 
that we are conscious of it only as an indistinct background; 
and when we direct our attention to the letters so that they are 
clear and legible, the threads of the veil become almost imper¬ 
ceptible and seem but as an intervening film, which scarcely 
interferes by its presence with the reading. In order to see 
sharply either one or the other, we are conscious of a change 
which we involuntarily make in the adjustment of the eyes. 

When a fly alights on a window-pane and our attention is 
attracted to it so that we see it clearly, the landscape beyond 
becomes indistinct and obscure; but when we look directly at 
the landscape so as to see its beauties, the fly on the pane 
becomes a shapeless spot. 

It is evident, therefore, from the foregoing illustrations 
that the eye cannot perceive distinctly and at the same time 
objects which are situated at different distances, but that it 
must fix alternately the nearer and the more remote, and ex¬ 
amine each in turn. It is also evident that in thus bringing 
alternately the one or the other object into distinct view, there 
must occur some change in the refractive power of the eye, by 
which the sight is adapted to the distance or nearness of the 
object under examination. 

The change which takes place as above described, or the 
power of variation and adjustment of the eye for vision at 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. II? 

different distances, is known as the Accommodation of the Eve , 
and it is being constantly brought into requisition. The 
method by which the accommodation of the eye is effected 
forms one of the most important parts of the physiology of 
siffht, and this power of adaptation of the eye to vision at 
different distances has received the most varied explanations. 

It is obvious that the effect might be produced in one of 
two ways: by an alteration of the convexity or intensity of 
either the cornea or crystalline lens, thus changing their refrac¬ 
tive power; or by changing the position of either the retina or 
lens, so that whether the object viewed be near or far, and the 
focal distance be diminished or increased, the focal point to 
which the rays are converged may always be at the place occu¬ 
pied by the retina. The amount of either of these changes 
required in even the widest range of vision is extremely small. 
It has been calculated that the difference between the focal dis¬ 
tances of the images of an object at infinite distance, and of one 
as close as four inches, is only about of an inch. 

In considering this subject it is well to remember that 
accommodation for distant vision is a passive condition, re¬ 
quiring no effort on the part of the eye, while accommodation 
for near objects is the result of muscular effort. This fact is, 
to some extent, made apparent by the nature of the sensations 
accompanying the change. The eye rests without fatigue for 
an indefinite time upon remote objects, while for the examina¬ 
tion of those close at hand a certain effort is necessary, and if 
it be prolonged, it, after a time, amounts to a sense of fatigue. 

It may also be remarked in passing that a solution of 
atropia (the active principle of belladonna) when applied to 
the eye causes temporary paralysis of the sphincter muscle of 
the iris and consequent dilatation of the pupil, and suspends 
more or less completely the power of accommodation for near 
objects, while in emmetropic eyes distant vision remains undis¬ 
turbed. Now it naturally follows that if accommodation for 
far and near objects was in each case the result of muscular 
action, the atropia that paralyzes one would also certainly 
paralyze the other, in which case distant vision would be as 
much blurred as near vision. Another point that might be 
mentioned as corroborative proof in this direction, is the 


Il8 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

diminution and loss of accommodative power that occurs in 
advanced life, when the accommodation for near objects be¬ 
comes more and more deficient* while distant vision is but 
little if any affected. 

It is now almost universally believed that the essential 
change upon which all the results of accommodation are 
directly dependent, is due to a varying shape of the crystalline 
lens, its front surface becoming more or less convex according 
to the distance of the object looked at. The nearer the objects 
approach, the more convex does the front surface of the lens 
become, the back surface scarcely changing its shape and hav¬ 
ing but little or no share in the production of the effect 
required. 



Accommodation, then, depends upon an increase in the 
convexity of the crystalline lens, and this increased convexity 
occurs chiefly upon its anterior surface, and this point can be 
illustrated by a little experiment which any one can perform 
for himself. If in a darkened room a candle flame be held a 
little to one side of a person’s eye, so that its rays fall some- 




THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 119 

what obliquely upon the cornea of the eye under observation, 
and at an angle of about thirty degrees with its line of sight, 
and if the observer will place himself on the opposite side at an 
equal angle with the line of sight, he will see three distinct 
reflected images of the candle flame. 

The first image, which is the brightest of all, is a small 
upright image reflected from the anterior convex surface of the 
cornea. The second image, which is also upright, but some¬ 
what larger and less distinct than the first, is the reflection 
from the anterior convex surface of the lens. The third image 
is tolerably distinct, but smaller and inverted, and is the reflec¬ 
tion from the posterior surface of the lens acting as a concave 
mirror, and therefore, like all concave mirrors, giving a 
reversed image. 

If the eye under observation be now made to change its 
point of sight from a distant to* a near object, the position of 
the eye-ball remaining fixed, the middle image becomes 
smaller and clearer and approaches the first one. 

This experiment of the candle flame images, in which the 
middle image changes its appearance and position when the 
eye changes its point of observation, proves that during accom¬ 
modation for near objects, the anterior surface of the lens, 
from which this second image is reflected, becomes more bulg¬ 
ing and approaches the cornea; while the curvature of the 
cornea and the posterior surface of the lens remain unchanged. 
The advance of the iris and pupil, in consequence of the pro¬ 
trusion of the anterior surface of the lens, can also be observed 
directly by looking into the eye from the side. 

The accommodation of the eye for near objects is there¬ 
fore produced by an increased refractive power of the lens, 
from its increased convexity or from the greater bulging of its 
anterior face. This has the effect of increasing the rapidity of 
the convergence of the rays passing through it, and conse¬ 
quently compensates for their greater divergence before enter¬ 
ing its substance. In the ordinary condition of ocular repose, 
when the eye is directed to distant objects, the rays coming 
from any point of such objects arrive at the cornea in a nearly 
parallel condition, and are then focused by the refractive media 
of the eye on the retina, as is shown in the following figure: 


120 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 



If now the eye be directed to a near point and the accom¬ 
modation thus be called into play, the crystalline lens increases 
its anterior convexity, and by this means the divergent rays 
proceeding from the near object are more strongly refracted, 
and are still brought to a focus on the retina as before, as is 
shown in the following figure: 



Vision for Near Objects. 


It thus becomes possible to fix alternately in distinct 
vision, objects at various distances in front of the eye. 

THE MECHANISM OF ACCOMMODATION. 

Of course, the crystalline lens has no inherent power of its 
own to contract, and therefore its changes in shape must be 
produced by some power from without; this power is supplied 
by the ciliary muscle, although it was formerly thought that 
the iris and the external ocular muscles assisted in the change 
of the adjustment of the eye. 

This ciliary muscle that produces the change, on account 
of its function is called the muscle of accommodation, and the 
manner in which this muscle acts to produce these changes is 
somewhat as follows: 

The act of accommodation has been shown to depend 
upon an increase in the convexity, and hence also in the refrac¬ 
tive power, of the crystalline lens. The lens, which has a cer¬ 
tain innate elasticity, is kept partly flattened by the action of 
the suspensory ligament on the capsule of the lens ; the ciliary 
muscle has such attachments that when it contracts the tension 
of the ligament and of the capsule is relaxed and diminished to 
a proportionate degree; the pressure being thus removed, the 













THE EYE OPTICAELY; OR, THE PHYSIOLOGY OF VISION. I 21 

lens then tends to assume its naturally more convex form. On 
the diminution or cessation of the action of the ciliary muscle 
the lens returns in a corresponding degree to its former flat¬ 
tened shape by virtue of the elasticity of its suspensory liga¬ 
ment. Thus it appears that the eye is usually and naturally 
focused for distant objects. 

In connection with this increase of the convexity of the 
lens in near vision, there occurs a corresponding contraction 
of the pupil; dilatation of the same taking place when the atten¬ 
tion is withdrawn from near objects and fixed on those at a 
distance. 

Our present more or less complete knowledge of the ac¬ 
commodation of the eye and of the mechanism by which it is 
accomplished, simple as it now seems to us, has been arrived 
at only after an immense amount of laborious work and 
patient research, one physiologist after another finding out 
and recording some new point bearing on the subject, until 
finally the master minds of Cramer and Helmholtz, utilizing the 
recorded results'of past investigations, and adding others of 
their own, finally developed the whole subject into the char¬ 
acter of an exact science. 

The effect produced by the effort of accommodation is 
exactly the same as would be produced by the placing of an 
additional convex lens within the eye. 

Accommodation is an involuntary act, and when we look 
at near objects it occurs almost without our consciousness, 
and yet it is none the less the effect of muscular effort; hence 
the relief that often follows from looking up and away from 
near work, and this is especially grateful to fatigued and sen¬ 
sitive eyes. 

When the ciliary muscle has contracted as much as it can, 
and the lens assumed the greatest convexity possible, then the 
maximum amount of accommodation is in force and the eye is 
adjusted for its near point. In every eye there is a limit to the 
power of accommodation; that is, there is a limit to the con¬ 
vexity which the lens is capable of assuming; and when this 
limit is reached a closer approximation of the object neces¬ 
sarily destroys the accuracy of its image. This is evidenced in 
the case of a book which is brought nearer and nearer to the 


122 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

eye, until at last the type becomes indistinct and cannot be 
brought into focus by any effort of accommodation, however 
strong. 

The near point of distinct vision is usually determined by 
placing in the patient’s hand the small test type, and noting 
the closest point at which it is possible for him to read it with 
each eye, separately. 

A more accurate and scientific test for determining the 
near point is as follows: Two small holes not more than a 
line apart are pricked in a card with a pin, and it is important 
that their distance from each other should not exceed the 
diameter of the pupil. The card is held close in front of the 
eye and a small needle viewed through the pin holes. At a 
moderate distance it can be clearly focused, but when brought 
nearer than a certain point the needle appears blurred and 
sometimes double. The point where the needle ceases to 
appear clear and single is the near point. Its distance from the 
eye can, of course, be readily measured. In ordinary normal 
eyes during the early or middle periods of life, accommodation 
fails and vision becomes indistinct when the object is placed 
nearer than five or six inches from the eye. Between the 
limits of five inches and infinite distance, the amount of accom¬ 
modation required varies with the distance, but not by any 
means in simple proportion to the variation of the distance. 
For instance, the change of accommodation necessary to 
clearly see objects situated respectively at six and twelve 
inches, is much greater than that required for the respective 
distances of twelve inches and twenty-four inches. The farther 
the object is situated from the eye, the less difference is pro¬ 
duced in the appreciable divergence of the rays proceeding 
from it by any additional increase of distance; and conse¬ 
quently less variation is required in the refractive condition of 
the eye to preserve the accuracy of the image. 

It has been generally found that no very sensible effort 
of accommodation is required for objects situated at any dis¬ 
tance beyond twenty feet from the observer, while within this 
distance the amount of accommodation necessary to preserve 
distinct vision increases rapidly as the object approaches the 
eve. 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. I23 

An eye which is capable of accommodating for distinct 
vision throughout the w r hole range included between five 
inches and infinite distance, is in this respect a normal eye, and 
is said to be emmetropic. 

When the muscle of accommodation has relaxed to its 
utmost, and the lens assumed the least convexity possible, 
then the minimum amount of accommodation is in force and 
the eye is adjusted for its far point. In this condition the eye 
is said to be in a state of repose. In the emmetropic 
eye the far point is situated at infinity; but for practical pur¬ 
poses in determining the accommodation, we measure it in 
inches by noting the farthest distance at which it is possible 
for the person to read the small test type with each eye 
separately. 

The distance between the near point and the far point is 
called the range of accommodation. 

As age advances the elasticity of the lens diminishes, the 
muscle of accommodation loses power, the range of accommo¬ 
dation is restricted, and the near point naturally and gradually 
recedes from the eye. Observation shows that these changes 
commence at a very early age, even in childhood. Infants 
often examine minute objects at very short distances, in a 
manner which would be quite impracticable for the adult eye. 
As the changes mentioned above continue to increase, the time 
arrives, between the ages of forty and fifty years, when the 
incapacity of accommodation for near objects begins to inter¬ 
fere with the ordinary occupations of life; when this occurs the 
eye is said to be presbyopic. The vision is still perfect for dis¬ 
tance, but it can no longer adapt itself for the examination of 
objects close at hand. The remedy for this (as will be fully 
explained when we come to presbyopia) is a convex lens in the 
form of spectacles, to supply the deficiency in the convexity 
of the lens of the eye. 

The following table gives the range of accommodation at 
different ages: 


Years. 

Range. 

Years. 

Range. 

1 n 

14. 

D. 

40. 



12. 

D. 

50. 

.2.50 D. 

. 

90 

.10. 

D. 

60. 

.1. D. 

30. 

. 7. 

D. 

75. 

.0. D. 










124 THE eye optically; or, the physiology of vision. 


CONVERGENCE. 

Convergence is the act of directing the visual axes of the 
two eyes to the same point at some near distance, and it is 
accomplished by the action of the internal straight muscles. 
The act of convergence is intimately associated with the act of 
accommodation, so that for every increase of the converg¬ 
ence there is a corresponding increase of the accommodation. 

The object of convergence is the directing of the yellow 
spot of each eye toward the same point, so that the rays from 
any one point may strike the same portion of the retina of each 
eye, producing a similar image on corresponding portions of 
the two retinae, and thus resulting in singleness of vision. 
This simultaneous use of both eyes is called “binocular 
vision”; and in order that it may be pleasant and satisfactory, 
the eyes should have the same refraction and the same acute¬ 
ness of vision, and both must be properly, directed to the same 
object. There is then an image formed on the retina of each 
eye, and the impression of the image carried to the brain by 
each optic nerve; but as the images are formed on correspond¬ 
ing parts of the two retinae, and as they are exact reproduc¬ 
tions—one of the other—they iare so combined by the brain as 
to give the impression of a single object. The advantages of 
binocular vision are the appreciation of solidity and the accu¬ 
rate determination of distance. 

Double vision at once results when the image of an object 
is formed on parts of the retinae which do not exactly corre¬ 
spond in the two eyes, because then the two images are so dis¬ 
similar that the brain is unable to fuse them into one. 

The nearer an object approaches to the eyes, the more 
strongly must they be converged, and the more the accommo¬ 
dation must be brought into play. Hence, in converging our 
eyes to any one particular point, we, at the same time, also 
involuntarily accommodate for the same point, as the converg¬ 
ence muscles (the internal recti muscles) and the accommoda¬ 
tion muscle (the ciliary muscle) act in unison. 

In order to test the power of convergence, prisms are 
used, and are placed before the eyes with their bases outward. 
The strongest prism which it is possible for the eyes to over- 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 12^ 

come—that is, the highest degree prism which does not pro¬ 
duce diplopia on looking through it at a distant object (such 
as a candle flame, which is the best object to use for such a 
test)—is the measure of the strength of the convergence. This 
varies in different persons, usually ranging between twenty 
degrees and thirty degrees—that is, a prism of from ten de¬ 
grees to fifteen degrees may be placed before each eye with 
their bases outward, and the strength of the convergence is 
sufficient to overcome the prisms and preserve singleness of 
vision. 

Prisms placed before the eyes with their bases in assist- 
convergence, and this fact is often made use of in the correc¬ 
tion of those cases of asthenopia due to an insufficiency of the 
internal recti muscles. 

When both eyes are directed simultaneously at a single- 
point, as is the case in binocular vision, the distance of the 
object may be estimated with considerable accuracy by the 
degree of convergence of the visual axes required for its fixa¬ 
tion. Since the degree of convergence required is in propor¬ 
tion to the proximity of the point of fixation to the observer, 
another impression of different kind, but of equal importance,, 
is also produced by binocular vision, when the object has an 
appreciable volume and thickness, and when it is placed within- 
a moderate distance. 

Owing to the lateral separation of the two eyes and the 
convergent direction of their visual axes, they do not both 
receive from the object precisely the same image. Both eyes* 
will see the front of the object in nearly the same manner, but,, 
in addition, the right eye will see a little of its right side and 
the left eye will see a little of its left side. This is illustrated 
in the following figure, which represents an outline cube: 



Illustrating the difference between the images seen by 
each eye separately. 



















126 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

If this cube be held at a moderate distance before the eyes, 
and viewed with each eye successively, while the head is kept 
perfectly steady, No. I will be the picture presented to the 
right eye, and No. 2 that seen by the left eye. 

As the central part of the object is in the point of fixation 
at the junction of the two visual axes, the object appears single. 
But the images which it presents to the two eyes, as is shown 
in the figure above, are not precisely identical in form; and on 
this circumstance depends, in a great measure, our conviction 
of the solidity of an object, or of its projection in relief; that 
is, it is the combination of these two different images into one 
which gives rise to the impression of solidity. 

If different perspective drawings of a solid body, one rep¬ 
resenting the image seen by the right eye, and the other that 
seen by the left (as in the above illustration), be presented to 
corresponding parts of the two retinae, as may be readily done 
by means of the stereoscope, the mind will perceive not merely 
a single representation of the object, but a body projecting in 
relief, the exact counterpart of that from which the drawings 
were made; the same optical effect is produced as by the 
object itself, and the appearance of solidity and projection are 
perfectly imitated. 

This is the principle of the instrument referred to above, 
which is known as the stereoscope. It is simply a framework 
holding two photographic views of the same object, which 
have been taken from two different points of view, correspond¬ 
ing to the different positions of the two eyes. 

Thus one of the pictures represents the object as it would, 
in reality, be seen by the right eye, and the other represents it 
as it would be seen by the left eye. When these two pictures 
are so placed in the stereoscope that each eye has presented to 
it its own appropriate view, the two images occupying the 
point of fixation are fused upon the retina, and produce an 
extremely deceptive resemblance to the projection and solidity 
of the real object. 

By transposing two stereoscopic pictures a reverse effect 
is produced; the elevated parts appear depressed, and the 
depressed parts appear to be elevated. Viewed in this way a 
bust would appear as a hollow mask. 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 12? 

But this combination of two different images into one, 
giving rise to the impression of solidity, is complete in effect 
only when the object is situated within a moderately short dis¬ 
tance. For those which are comparatively remote, the con¬ 
vergence of the visual axes, and consequently the difference in 
the apparent configuration of the two images, is so little as to 
become inappreciable, and the optical impression of solidity 
disappears. At a distance of some miles even a large object 
like a mountain loses its projection, and presents the form of a 
flattened mass against the horizon. It is on. this account that 
pictorial representations of distant views can often be made 
-extremely effective; the idea of successive remoteness in differ¬ 
ent parts of the landscape being conveyed by appropriate in¬ 
tersection of the outlines, and by variations in tone, color and 
distinctness, like those due to the interposition of the atmos¬ 
phere. 

On the other hand, a picture of near objects, which aims 
to represent their solidity, can never deceive us in this respect, 
however elaborate may be the details of surface, shadow and 
color; since the flat surface of the picture presents the same 
image to both eyes, and it is consequently evident that the 
objects delineated have no real projection. 

APPARENT POSITION OF OBJECTS. 

The apparent position of an object is determined by the 
direction in which the luminous rays pass from it to the in¬ 
terior of the eye. The perception of the light itself neces¬ 
sarily marks the direction from which it has arrived, and, 
therefore, the apparent position of its source. The result is 
that a ray coming from below attracts attention to the inferior 
part of the field of vision, while one coming from above is 
referred to its point of origin in the upper part of the same field. 

Thus if two luminous points appear simultaneously in the 
field of vision, they present themselves in a certain position 
with regard to each other, above or below, to the right or to 
the left, according to the direction in which their light has 
reached the eye. This fact is fully demonstrated by the phe¬ 
nomena of angular reflection and refraction. If a candle be 
held behind the back in such a position as to be reflected in a 


128 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

mirror placed at the front, the light presents itself to the eye 
as if it were really in front, because it is from this direction 
that the luminous rays finally come. 

If we observe the reflection of objects in a mirror held 
horizontally, or in a smooth sheet of water, the objects seem to 
be placed below the reflecting surface (although we know they 
are really above it), since the rays which make their impres¬ 
sion upon the eye actually come from below. A stick or peb¬ 
ble seen obliquely at the bottom of a transparent pond appears 
nearer the surface than it really is, because the rays which 
reach the eye have been bent from their course, in passing 
from the water into the atmosphere, and have consequently 
assumed a more oblique direction. 

The acuteness of vision, so far as it is connected with the 
sensibility of the retina, depends upon the smallest distance 
from each other of two visual rays, at which they can still be 
perceived as distinct points. If the luminous rays coming 
respectively from the top and bottom of an object are so 
closely approximated where they strike the retina that the two 
impressions are confounded, there can result no distinct im¬ 
pression or perception of its figure or dimensions. 

On the other hand, if the rays are far enough apart, and 
the sensibility of the retina be such that the two impressions 
are perceived as separate from each other, the form of the 
object will be recognized, as well as its luminosity, notwith¬ 
standing the small size of the retinal image. 

The figure of a man six feet high seen at a distance of ten 
yards, forms at the cornea a visual angle of n° 30', and pro¬ 
duces upon the retina an image which is less than one-fiftieth 
of an inch in length; and yet an abundance of details are dis¬ 
tinctly perceptible within this space. 

The extreme limit of approximation at which two points 
may still be distinguished from each other has been examined 
by the observation of fixed stars, and of parallel threads of the 
spider’s web, or of fine metallic wires placed at known dis¬ 
tances from each other. The general result of these exami¬ 
nations has shown that for the average of well-formed eyes the 
smallest visual angle at which two adjacent points or lines can 
be distinguished is from sixty to seventy-three seconds. 


THE EYE OPTICALLY ; OR, THE PHYSIOLOGY OF VISION. 1 29 

The duration of the sensation produced by a luminous 
impression upon the retina is always greater than that of the 
impression that produces it. However brief the luminous 
impression, the effect on the retina always lasts for about one- 
eighth of a second. Thus supposing an object in motion, say 
a horse, be revealed on a dark night by a flash of lightning. 
The horse would be seen apparently for an eighth of a second, 
but it would not appear in motion; because, although the 
image remained on the retina for this time, it was really re¬ 
vealed for such an extremely short period (a flash of lightning 
being almost instantaneous) that no appreciable motion on the 
part of the object could have taken place in the period during 
which it was revealed to the retina of the observer. 

And the same fact is proved in a reverse way. The spokes 
of a rapidly revolving wheel are not seen as distinct objects, 
because at every point of the field of vision over which the re¬ 
volving spokes pass, a given impression has not faded before 
another comes to replace it. Thus every part of the interior 
of the wheel appears occupied. 

It is quite evident that the more luminous a body, the 
more intense is the sensation it produces. Part of the light 
which enters the eyes is absorbed and produces certain 
changes in the retina, while the rest is reflected. 

Every one is perfectly familiar with the fact that it is quite 
impossible to see the fundus or back part of another person’s 
eye by simply looking into it. The interior of the eye forms a 
perfectly black background to the pupil. The same remark 
applies to an ordinary photographer’s camera, and may be 
illustrated by the difficulty we experience in seeing into a 
room from the street through a window, unless the room be 
lighted within. 

In the case of the eye this fact is partly due to the feeble¬ 
ness of the light reflected from the retina, most of it being 
absorbed by the choroid, as has been mentioned; but far more 
to the fact that every such ray is reflected straight back to its 
source, and cannot therefore be seen by the unaided eye with¬ 
out intercepting the incident light from the candle as well as 
the reflected rays from the retina. This difficulty has been 
surmounted by the ingenious device of Helmholtz, which is 


130 THE EYE OPTICALLY ; OR, THE PHYSIOLOGY OF VISION. 

now so extensively used as the opthalmoscope. It consists of 
a small slightly concave mirror, by which light is reflected 
from a candle into the eye. The observer looks through a 
hole in the mirror and can thus explore the illuminated fundus, 
the entrance of the optic nerve and the retinal vessels being 
plainly visible. 

The method by which a ray of light is able to stimulate 
the endings of the optic nerve in the retina, in such a manner 
that a visual sensation is produced and perceived by the brain, 
is not yet fully understood. 



The apparatus of vision, as it has been described, consists 
of various parts, each of which has its appropriate share in 
producing the final result of visual perception. The eye, as 
regards its physical structure, is an optical instrument, 
as has already been stated, composed of transparent and re¬ 
fracting media, a perforated diaphragm, and a dark chamber 
lined with a blackened membrane, all of which act upon the 
luminous rays according to the same laws as the corresponding 
parts in a telescope or camera; and the.accuracy of the adjust¬ 
ment of these structures is one of the first requisites for the ex¬ 
ercise of sight. The eye, furthermore, is movable as a whole; 
and certain of its internal parts are also under the control of 
muscular tissues, the alternate contraction and dilatation of 
which contribute to determine its mode of action. It is in 
addition a double organ, and impressions may be derived from 
the simultaneous employment of both eyes which can not be 
acquired by the use of one alone. 

Rays of light entering the eye from the front, in the line 
of direct vision, may be brought to an accurate focus at the 
situation of the retina. But those which enter at a certain 
degree of obliquity, whether from above or below, from one 



THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. I 3 I 

side or the other, suffer a more rapid convergence, and are 
accordingly brought to a focus and again dispersed before 
reaching the retina. 

Thus rays diverging from the point A in the line of direct 
vision are concentrated at X , and form a distinct image upon 
the retina at that point. 

But those proceeding from the point B, which is situated 
considerably to one side, under a similar degree of divergence, 
fall upon the cornea and crystalline lens in such a way that 
there is more difference in their angles of incidence, and con¬ 
sequently more difference in the degree of their refraction. 
They are therefore brought together too rapidly and are dis¬ 
persed upon the retina over the space Y Z, forming an imper¬ 
fect image. 

Opthalmoscopic examination of the retina shows that the 
images formed at the fundus of the eye from luminous objects 
in the line of direct vision present perfectly distinct outlines, 
while those at a certain distance from this point toward the 
lateral parts of the retina are comparatively ill-defined. 

ESTIMATION OF SIZE. 

Our estimate of the size of various objects is based partly 
on the visual angle under which they are seen, but much more 
on the estimate we form of their distance from us. Thus a 
lofty mountain, many miles away, may be seen under the same 
visual angle as a small hill near at hand, but we infer that the 
former is much the larger object because we know that it is 
much farther off than the hill. But it often happens that our 
estimate of distance is erroneous, and consequently our esti¬ 
mate of size will also often be faulty. Thus persons seen 
walking on the top of a small hill against a clear twilight sky 
appear unusually large, because we over-estimate their dis¬ 
tance; and for similar reasons most objects in a fog appear 
immensely magnified. 

The same mental process gives rise to the idea of depth in 
the field of vision. The action of the sense of vision, in rela¬ 
tion to external objects, is therefore quite different from that 
of the sense of touch. The objects of the latter sense are im¬ 
mediately present to it; and our own body with which the\ 


132 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

come in contact is the measure of their size. In the sense of 
vision, on the contrary, the images of objects realized upon the 
retina are mere fractions of the objects themselves, the extent 
of the retina always remaining the same. But the imagina¬ 
tion, which analyzes the sensations of vision, invests the images 
of objects with varying dimensions, the relative size of the 
image in proportion to the whole field of vision remaining 
unaltered. 

The estimation of the direction in which an object is seen 
depends on the part of the retina which receives the image, and 
on the distance of this part from the central point of the retina 

The estimation of the form of bodies by sight is the result 
partly of the mere sensation and partly of the association of 
ideas. 

The clearness with which an object is perceived, irrespec¬ 
tive of accommodation, would appear to depend largely on the 
number of rods and cones which its retinal image covers. 
Hence the nearer an object is brought to the eye (within 
moderate limits) the more clearly are all its details seen. More¬ 
over, if we want to examine any object carefully, we always 
direct the eyes straight to it, so that* its image shall fall on the 
yellow spot, where an image of a given area will cover a larger 
number of cones than anywhere else in the retina. The diam¬ 
eter of each cone in this part of the retina is about one twelve- 
thousandth of an inch, and consequently it has been found that 
the images of two points must be at least one twelve-thou¬ 
sandths of an inch apart on the yellow spot in order to be dis¬ 
tinguished separately; if the images are nearer together the 
points appear as one. 

ERECT VISION WITH AN INVERTED IMAGE. 

Since it is the direction of the visual rays, rather than the 
point of their impact upon the retina, which determines the 
apparent relative position of luminous objects, such objects 
appear erect even though their images upon the retina are 
inverted. The image formed upon the retina is not the form 
which is seen by the eye and recognized by the brain, but this 
retinal image is only a phenomenon visible to the inspection of 
another eye. 


i'HE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 133 

It is desirable that every optician should know that the 
image that is formed upon the retina is in an inverted position, 
and this at once very naturally raises the question as to why 
we do not, therefore, see everything upside down. This is a 
subject on which much has been said and written, and many 
ingenious explanations have, from time to time, been ad¬ 
vanced, and it has become really quite a bugbear to optical 
students. The question briefly stated is this: How is it pos¬ 
sible that we see objects erect, when the images formed bv 
them upon the retina are inverted? 



The above figure represents the formation of an inverted 
image upon the retina of the eye. There has been much 
learned discussion as to the manner in which we receive the 
impression of an erect object from an inverted image. 

It should be remembered that after all it is not the eye 
that sees, but the brain, and it sees, not the image formed upon 
the retina, but what is called the projection outward of this 
image, just as the picture of the magic lantern slide, which is 
placed in the lantern upside down, projects an erect image 
upon the screen. In other words, when the inverted image is 
formed upon the retina, we refer the sensation in the same 
direction as the rays that produce it; hence, in that part of the 
image that is formed on the upper part of the retina, we refer 
the sensation downward along the line from which its rays 
must have come; and in like manner that part of the image 
that is formed on the lower part of the retina is referred up¬ 
ward; hence, the image rectifies itself. 

To explain more minutely: The direction given to rays 
by their refraction is regulated by that of the central ray or 
axis of the cone toward which the rays are bent. The image 






134 THE EYE optically; or, the physiology of vision. 

of any point of an object is, therefore, as a rule, always formed 
in a line identical with the axis of the cone of light, as in the 
line B a, or A b, so that the spot where the image of any point 
will be formed upon the retina may be determined by prolong¬ 
ing the central ray of the cone of light, or that ray which 
passes through the nodal point. 



Thus A b is the axis or central ray of the cone of light 
issuing from A; B a, the central ray of the cone of light issu¬ 
ing from B. The image of A is formed at b, the image of B 
at a, in the inverted position; therefore, what in the object 
was above, is in the image below, and vice versa. The right- 
hand part of the object is in the image to the left, the left-hand 
to the right. 

If an opening could be made in an eye at its upper surface, 
so that the retina could be seen through the vitreous humpr, 
this reversed image of any bright object, such as the windows 
of a room, would be perceived at the bottom of the eye; or still 
better, if the eye of any albino animal, such as a white rabbit, 
in which the coats, from the absence of pigment, are transpar¬ 
ent, is dissected clean and held with the cornea toward the 
window, a very distinct image of the window, completely 
inverted, will be seen depicted on the posterior translucent 
wall of the eye. 

An image formed at any point on the retina is referred to 
a point outside the eye, lying on a straight line drawn from the 
point on the retina outward through the center of the pupil. 
Thus an image on the left side of the retina is referred by the 
mind to an object on the right side, and vice versa. Thus all 
images on the retina are mentally, as it were, projected in front 
of the eye, and consequently all objects are seen erect even 
though the image on the retina is reversed. 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. I35 

Much needless difficulty and confusion have been raised 
on this subject, for want of remembering that when we are 
said to see an object, the mind is merely conscious of the 
picture on the retina, and when it refers it to the external 
object, or projects it outside of the eye, it necessarily reverses 
the picture and sees the object erect, while at the same time 
the retinal image is inverted. This is further corroborated by 
the sense of touch; thus an object whose picture falls on the 
left half of the retina is reached by the right hand, and hence 
is said to lie to the right; or again, an object whose image is 
formed on the upper part of the retina is readily touched by 
the feet, and is, therefore, said to be in the lower part of the 
field of vision. 

Hence it is, also, that no discordance arises between the 
sensations of inverted vision and those of touch, which per¬ 
ceives everything in its erect position, for the images of all 
objects, even of our own hands and feet, are equally inverted 
on the retina, and therefore maintain the same relative position. 
The position in which we see objects we call the erect posi¬ 
tion. A mere lateral inversion of our body in a mirror, where 
the right hand occupies the left of the image, is, indeed, 
scarcely remarked; and there is but little discordance between 
the sensations acquired by touch in regulating our movements 
by the image in the mirror and those of sight, as, for example, 
in tying a bow in a cravat. The perception of the erect posi¬ 
tion of objects appears, therefore, to be the result of an act of 
the mind. 


FIELD OF VISION. 

The actual size of the field of vision depends upon the 
extent of the retina, for only so many images can be seen at 
any one time as can occupy the retina at the same time; and 
thus considered, the retina, the images on which are perceived 
by the mind, is itself the field of vision. But to the mind of the 
individual, the size of the field of vision has no determinate 
limits; sometimes it appears very small, at other times very 
large. The mental field of vision is very small when the sphere 
of the action of the mind is limited to impediments near the 
eye; on the contrary, it is very extensive when the projection 


136 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

of the images on the retina, toward the exterior, by the influ¬ 
ence of the mind is not impeded. It is very small when we 
look into a hollow body of small capacity held before the eyes; 
it is large when we look out upon a landscape, through a small 
opening; more extensive when we view a landscape through a 
window; and most so when our view is not confined by any 
near object. 

In all these cases the idea which we receive of the size of 
the field of vision is very different, although its absolute size is 
the same in all because it is dependent upon the extent of the 
retina. Hence it follows that the mind is constantly cooperat¬ 
ing in the acts of vision, so that at last it becomes difficult to 
say what belongs to mere sensation and what to the influence 
of the mind. By a mental operation of this kind we obtain a 
correct idea of the size of individual objects, as well as of the 
extent of the field of vision. This is illustrated in the following 
figure: 



The angle x, included between the decussating central 
rays of the two cones of light issuing from different points of 
an object, is called the optical angle. This angle becomes 
larger the greater the distance between the points A and B, 
and since the angles x and y are equal, the distance between 
the points a and b, in the image on the retina, increases as the 
angle becomes larger. 

Objects at different distances from the eye, but having the 
same optical angle x (for example, the objects c, d and e), must 
also throw images of equal size upon the retina; and if they 
occupy the same angle of the field of vision, their images must 
occupy the same spot on the retina. Nevertheless these 
images appear to the mind to be of very unequal size when the 





THE EYE OPTICALLY ; OR, THE PHYSIOLOGY OF VISION. 137 

idea of distance and proximity comes into play, for from the 
image a b the mind forms the conception of a visual space 
extending to e, d or c, and of an object of the size which that 
represented by the image on the retina appears to have when 
viewed close to the eye, or under the most usual circumstances. 

ESTIMATION OF MOVEMENT. 

We judge of the motion of an object partly from the 
motion of its image over the surface of the retina, and partly 
from the motion of our eyes necessary to follow it. If the 
image upon the retina moves while our eyes and our body are 
at rest, we conclude that the object is changing its relative 
position with regard to ourselves. If, on the other hand, the 
image does not move with regard to the retina, but remains 
fixed upon the same spot of that membrane, while our eyes 
follow the moving body, we judge of its motion by the sensa¬ 
tions of the muscles called into action to move the eye. If the 
image moves over the surface of the retina while the muscles 
of the eye are acting at the same time in a direction corre¬ 
sponding to this motion, as in reading, we infer that the object 
is stationary, and we know that we are merely altering the rela¬ 
tion of our eyes to the object. Sometimes the object appears 
to move when object and eyes are both fixed, as in vertigo. 

COLOR SENSATIONS. 

If a ray of sunlight be allowed to pass through a prism it 
is decomposed by the prism into rays of different colors, which 
are called the colors of the spectrum. They are red, orange, 
yellow, green, blue, indigo and violet. The red rays are the 
least turned out of their course by the passage through the 
prism, and the violet rays the most, whilst the other colors 
occupy in their respective order places between these two 
extremes. The difference in the color of the rays depends 
upon the number of vibrations producing each, the red rays 
being the least rapid, and the violet the most. These colored 
rays, which are perceived by the brain as such, must stimulate 
the retina in some special manner in order that colored vision 
may result, and two chief explanations of the method of this 


138 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

stimulation have been suggested, but these theoretical specu¬ 
lations are of so little practical value that it will not pay us to 
devote any space to their consideration. 

The ocular spectra, which follow the impression of 
colored objects upon the retina, are also always colored; but 
their color is not the same as the object, or of the image pro¬ 
duced directly by the object, but the opposite or complemental 
color. The spectrum of a red object is, therefore, green; that 
of a green object, red; that of a violet, yellow; that of a yellow, 
violet, and so on. The reason of this is obvious; the part of 
the retina which receives a certain color, say a red image, is 
wearied by that particular color, but remains sensitive to the 
other rays which, with red, make up white light; and, there¬ 
fore, these by themselves, reflected from a white object, pro¬ 
duce a green hue. If, on the other hand, the object first 
looked at be green, the retina, being tired of green rays, 
receives a red image when the eye is turned to a white object. 
And so with other colors; the retina, while fatigued by yellow 
rays, will suppose an object to be violet, and vice versa. Of 
course the size and shape of the spectrum always correspond 
with the size and shape of the original object looked at. 

Color-blindness is a by no means uncommon visual defect. 
One of the commonest forms is the inability to distinguish 
between red and green; the explanation of this is that the 
elements of the retina which receive the impression of red are 
absent or very imperfectly developed. Color-blindness is a 
most interesting subject, and we hope later on to give some 
attention to it, but just now other subjects, more practical and 
important, are pressing for attention. 

SINGLE VISION WITH TWO EYES. 

Although the sense of sight is exercised by two organs, 
yet the impression of an object conveyed to the mind is single. 
Various theories have been advanced to account for this phe¬ 
nomenon. By some authorities it has been supposed that we 
do not really employ both eyes simultaneously in vision, but 
see with only one at a time. This especial employment of one 
eye only in vision certainly occurs in persons whose eyes are 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 139 

of very unequal refraction, but in the majority of individuals 
both eyes are simultaneously in action in the perception of the 
same object. This is proven by the double images seen under 
certain conditions. 

If two fingers be held before the eyes, one in front of the 
other, and vision be directed to the more distant, so that it is 
seen singly, the nearer one will appear double; while if the 
nearer one be regarded, the distant one will be seen double; 
and in each case one of the double images will be found to 
belong to one eye, the other image to the other eye. 

As has already been stated, distinct vision is possible for 
an eye only for objects situated in the line of direct vision. 
Now, since the eyes are placed in their orbits at a certain dis¬ 
tance from each other, when they are both directed at the same 
object their lines of direct vision converge and cross each other 
at a single point. At this point of intersection of the two lines 
of direct vision an object may be seen distinctly by both eyes 
at the same time. But at every other point it must appear in¬ 
distinct to one of them; there is, therefore, only a certain dis¬ 
tance directly in front at which an object can be distinctly seen 
simultaneously by both eyes, namely, at that point where the 
two lines of direct vision intersect each other. This point is 
called the point of fixation for the two eyes. In fixing any 
object for binocular vision, the accommodation in each eye is 
at the same time adjusted for the required distance, and thus 
the entire accuracy of both organs is concentrated upon a 
single point. 

Since it is the position of the two eyes in their respective 
orbits which determines the point of fixation, the observer can 
form a tolerably accurate judgment as to whether another per¬ 
son within a moderate distance can be looking at him, or at 
some other object farther removed in the same direction. 

From the preceding facts it is evident that only one point 
can be found in the line of direct vision for both eyes at the 
same time. When an object occupies this situation it is dis¬ 
tinctly perceived by both eyes in the center of the field of 
vision. Thus its two visual images exactly cover each other in 
their apparent position, and so form but one. Consequently 
the object appears single, though seen by both eyes. But if 


140 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

placed either within or beyond the point of fixation, the object 
appears indistinct, and at the same time double. 



Single and double vision at different distances, a, right eye. ft, left eye. 
1, object at the point of fixation, seen single. 2, object 
beyond the point of fixation, seen double. 


When the eyes are so directed that the nearer object (i) 
occupies the point of fixation, the farther object (<?) will also 
be seen, because it is still included in the visual field; but it 
will be seen indistinctly, because the accommodation of the 
eye is no longer adjusted to its distance, and because it is not 
in the line of direct vision. But for the right eye (a) it will be 
placed to the right of this line, and for the left eye ( b) to the 
left of it. Its two images do not correspond with each other 
in situation, and the object accordingly appears double. 

If the eyes, on the other hand, be directed to the more dis¬ 
tant object, the nearer one is no longer in the point of fixation. 
For the right eye its image will appear to the left of the line of 
direct vision, and for the left eye, to the right of this line, and 
it, therefore, appears double and indistinct. 

Thus in the ordinary use of binocular vision every object 
but one appears double, and at the same time imperfectly 
delineated. This circumstance is so little noticed that it is 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 141 

never a source of confusion for the sight, but even requires a 
special experiment to demonstrate its existence. The reason 
for its passing unobserved is twofold; first, the attention is 
naturally concentrated upon the object which is placed for the 
moment at the point of fixation. When this point is shifted, 
the new object upon which it falls also appears single, and thus 
the idea of a double image, even if indistinctly suggested at 
any time, is at once dispelled by the movements of the eyes in 
that direction. 

In the second place, an object which is really placed in 
any degree toward the right-hand or the left will form an 
indistinct double image, since it occupies a different apparent 
position for the two eyes. But the obliquity of its rays, and 
consequently the indistinctness of its image, will be greater for 
the right eye than for the left, and vice versa; and the notice 
of the observer, if drawn to it at all, is occupied with the more 
distinct of the two images to the exclusion of the other. 

Double vision may also be produced at any time by pres¬ 
sure with the finger at the external angle of one of the eyes, so 
as to alter its position in the orbit, the other eye remaining 
untouched. But in this case it is the whole field of vision that 
is displaced, and all objects are doubled indiscriminately; their 
images being separated to the same degree and in the same 
direction, whatever may be their distance from the eye. It is 
this form of double vision which is produced in vertigo or 
intoxication, by irregular action of the muscles of the eye-ball. 

The sensibility of the retina is diminished by continued 
visual impressions, and this fact becomes apparent by the fol¬ 
lowing test: If one eye be covered by a dark glass and the 
other be used exclusively for an hour or two in reading or 
writing, at the end of that time the difference in the retinal 
sensibility of the two eyes will be marked. A single faintly 
luminous object in a dark room may then be almost impercep¬ 
tible to the eye that has been exhausted by the hour’s use, 
while it will appear quite brilliant to the other eye. If the 
application of the eye has not been carried beyond the bounds 
of moderation, this difference is transitory. By reversing the 
conditions, that is, by covering the eye previously in use, and 
reading or writing by the other, that which before was the 


142 THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. 

most sensitive to light becomes less so, and that which was 
previously fatigued recovers its sensibility. 

The alternate diminution and recovery of the retinal sen¬ 
sibility by excitement and repose is directly connected with the 
phenomenon of negative images. If the eye be steadily fixed 
for a short time upon a white spot in the center of a black 
ground, and then suddenly directed toward a blank wall of a 
uniform white or light gray color, a dark spot will appear at 
its center, of the same apparent size and figure as the white 
■one previously observed. This is the “negative image” of the 
retinal impression. That part of the retina which was first 
impressed by the rays from the white spot becomes less sen¬ 
sitive to light, and consequently another white surface, 
looked at immediately afterward, appears darker than usual. 
On the other hand, those parts which were exposed only to 
the dark ground, that is, to the comparative absence of light, 
.are more sensitive than before, and the surface of the white 
wall outside the central dark spot appears brighter than usual. 

In further illustrating the phenomenon of negative im¬ 
ages, let a black ruler about an inch wide be laid upon a sheet 
of white paper, and looked at steadily for thirty or forty 
seconds. If the ruler be now removed by a sudden motion, the 
eye remaining fixed, its image will appear as a bright band 
upon the paper, fading gradually as the sensibility of the retina 
becomes equalized in its different parts. 

If the figure which is thus examined be a colored one, its 
negative image, subsequently produced, will present a comple¬ 
mentary hue to that of the original object. A strip of red 
paper placed upon the white sheet and then suddenly removed, 
leaves a negative image which is bluish-green; and a green 
strip of paper leaves an image which is decidedly reddish. 
This shows that the sensibility of the retina may be increased 
-or diminished separately for the different colored rays of the 
luminous beam. While looking at the red object, the retina 
partly loses its sensibility for the red rays while increasing it 
for those at the opposite end of the spectrum, and vice versa; 
so that on looking subsequently at a white object, the negative 
image exhibits a tint corresponding to the rays for which the 
retina has remained most sensitive. 


THE EYE OPTICALLY; OR, THE PHYSIOLOGY OF VISION. I 43 

That this is the mechanism of the production of comple¬ 
mentary colors in negative images becomes evident by another 
experiment. If the black ruler be laid upon a blue surface, 
the band which remains in its place after taking it away is of 
a more intense blue than the rest. If a red surface be used for 
the same purpose, the negative image of the ruler presents a 
remarkably pure red color, while the remainder of the surface 
appears of a dull brown. 

The variable sensibility of the retina, according to its 
exposure, affords an explanation of the well-known fact that, 
under some conditions, an object may be most easily perceived 
by indirect vision. It often happens that in searching for a 
star of very small magnitude and feeble light, it may be mo¬ 
mentarily perceived by looking not directly at it, but at a point 
in its immediate vicinity, at a small angular distance from its 
real position. The star is not seen distinctly under these cir¬ 
cumstances, because it is out of the line of direct vision; but 
its light falls upon a part of the retina near the yellow spot, 
the sensibility of which is more acute than usual, owing to its 
continued exposure only to the dark sky; while the yellow 
spot itself, which has been receiving in succession the images 
of particular stars, is comparatively deficient in impressibility 
to light. When the visual axis is turned directly upon the 
fainter star for the purpose of getting a distinct image, its light 
disappears; and thus it can only be seen as an evanescent 
object by indirect vision. 

If the eye be fixed immovably for too long a time upon 
the same luminous object, the local diminution of retinal sensi¬ 
bility may amount to fatigue; and a persistence in its continu¬ 
ous application may produce permanent injury to the visual 
organ. 

After steadily examining a single object for even a short 
time, it becomes difficult to resist the tendency to turn the 
sight in another direction by the automatic movement of the 
muscles of the eye-ball. Naturally, the eye never rests for 
more than a few seconds upon any one point in the field of 
view, but is directed in succession at different objects, fixing 
each' one in turn at the point of distinct vision and immedi¬ 
ately passing to another more or less remote. Thus fatigue 


144 the eye optically; or, the physiology of vision. 

of the retina is avoided, since those parts which, at one instant, 
have a stronger illumination, at the next instant receive the 
impression of a shadow; and no portion of the membrane is 
exposed sufficiently long to any single object to become insen¬ 
sible to its grade of light or color. 

There is, also, reason to believe that the eye requires for 
its safety a periodical suspension of all visual impressions, 
such as is obtainable only in sleep. It is not essentially differ¬ 
ent in this respect from other parts of the nervous apparatus of 
animal life; but the delicacy of its sensibility, which is requi¬ 
site for the due performance of its function, and the complica¬ 
tion of its structure, which includes so many parts adjusted to 
each other with mathematical accuracy, indicate that it is one 
of the organs most liable to derangement if deprived of its 
natural interval of restoration and repose. 

We have now gone over the anatomy of the eye and the 
physiology of vision, covering the ground pretty carefully. 
Much of what has been said above may have seemed somewhat 
tedious and uninteresting, but it is important knowledge for 
the optician to possess as a foundation for the more practical 
matter which is to follow. 


CHAPTER VII. 


THE USE AND VALUE OF GLASSES. 


Every one knows, in a general way, that spectacles are 
worn to assist the sight, and there are a large number of per¬ 
sons who know nothing more than that they help the aged to 
see to read, and the near-sighted to see at a distance. The 
prejudice against glasses has, on the one hand, acted to deter 
persons from wearing glasses who really need them; and, on 
the other hand, has acted to influence employers to decline to 
engage applicants for work who are spectacled. 

It is hardly within the scope of this work, at the present 
time, to attempt to combat these prejudices or to advance any 
arguments to prove their fallacy. The first one is so unreason¬ 
able that no sensible person in this enlightened day would be 
rash enough to advise ametropic persons not to wear glasses, 
nor would the ametrope be so foolish as to listen to such 
advice. While in regard to the second prejudice mentioned, 
employers are soon and easily convinced that the man who 
notices his sight is failing, and promptly uses the means at his 
command to restore it, is the more wide-awake and capable 
workman, and does better service than the slothful man who 
fails to notice his impaired sight, or, if he does, takes no in¬ 
terest or makes no effort to remedy it. The truth of the matter 
is that, in the hands of a skilful optician, there is no means at 
the present time which will correct so many eye troubles and 
restore good sight as suitable lenses in all their various combi¬ 
nations, and people have not been slow to find this out and to 
act in accordance with it. 

The common ignorance in regard to glasses and their uses 
has bred the idea that persons can choose their own glasses; 
but the fallacy of supposing that the glasses that are the most 
pleasant for two or three minutes are necessarily the best to 
use for years, is not so general now as formerly. 

145 



146 


THE USE AND VALUE OF GLASSES. 


Not every one suffers from such a course, and yet the 
great number that do makes it safer and advisable, in order to 
avoid any risk, to have the eyes tested by a competent optician 
in even the simplest cases. 

Experience has taught the public much in this respect (of 
the importance of the proper selection of glasses); but there 
are still many persons who would decidedly object to wearing 
ill-fitting ready-made clothing, who still do not hesitate, in the 
infinitely more- important and delicate matter of selecting 
glasses, to purchase them in the old haphazard way without a 
measurement by oculist or optician, without thinking of the 
danger of thus treating an organ whose mechanism is of 
the most delicate nature, and whose use is almost as valuable 
as life itself. 

Optical defects requiring the use of glasses may be classi¬ 
fied as follows: 

1. Presbyopia, or old sight; that condition of the eye where 
there is a deficiency in the power of accommodation of the eye, 
due to weakening of the ciliary muscles and to hardening of 
the crystalline lens from age. 

2. Hypermetropia, or far-sightedness; the eye is too flat, 
and its refraction is not sufficient to bring parallel rays of light 
to a focus on the retina. 

3. Myopia, or near-sightedness; the eye is too long or 
deep, and its refraction causes parallel rays of light to focus 
before they reach the retina. 

4. Astigmatism, or irregular sight; due to an irregularity 
of the surface of the cornea and a difference in the refraction 
of its different meridians. 

5. Asthenopia, or weak sight; due to a weakness of the 
ciliary muscle or of the recti muscles. 

6. Diplopia, or double sight; where the two eyes cannot 
be directed to the same object. 

7. A large class of cases where the various colored (non- 
focal) glasses are required. 

8. Another class of cases (not necessarily of optical defect) 

in workmen, where glasses are needed to protect the eyes from 
mechanical injury. j 


THE USE AND VALUE OF GLASSES. 


147 


Presbyopia and hypermetropia are corrected by convex 
:spherical lenses; astigmatism by convex or concave cylindrical 
lenses; asthenopia and diplopia by a suitable adjustment or 
combination of one or more of the above-mentioned kinds, 
■with or without the further combination of prismatic lenses. 

Were these results all that could be expected from glasses, 
their value would be inestimable. But in addition to these 
optical defects, a great many diseases, dangerous to vision, and 
dependent upon or caused by these errors of refraction and 
accommodation, are relieved and cured by the adjustment of 
the proper lenses. This opens a wide and almost illimitable 
field of usefulness—a field so extended that it may fairly 
occupy the undivided time and special study of the most dis¬ 
tinguished oculists, and yet in which the educated optician can 
work with satisfaction to his customers and profit to himself. 

In that large class of cases where colored glasses are 
needed, it should be remembered that while no one seems to 
have any suspicion that such glasses could be in any manner 
injurious, yet it is essential that great care be exercised in 
selecting only those which are known certainly to have no 
focus; otherwise they only increased the irritability of the eye 
instead of allaying it, especially if the cheap coquilles are 
worn, as these, being made of moulded or pressed glass, are 
seldom without a focus (which is generally concave), in addi¬ 
tion to which the glass is of poor quality and full of flaws and 
imperfections, which produce an unpleasant distortion of ob¬ 
jects seen through them. 

Unless specially desired to correct some error of refraction 
(in which case the proper focus is ground on a colored lens), 
tinted glasses should be absolutely without focus, and should 
not distort or dim objects that are seen through them. The 
simplest way to determine whether any colored glass is plane 
or has a focus, is by holding it up to the light and slowly 
moving it up and down and from side to side while the 
window-sash or any other stationary object is looked at. If 
it is a plane glass, and without focus, no effect will be produced 
on the window-sash by the motion of the glass. On the other 
hand, if a professedly plane glass is not free from refraction, a 
motion of the window-sash will be produced by the movement 


148 


THE USE AND VALUE OF GLASSES. 


of the lens, either with it or against it, proving it to be in the 
first case concave, and in the second convex. If there is any 
such movement, or if the objects are seen imperfect and dis¬ 
torted, the glass should be rejected as not a safe one to wear. 

When coquille glasses have a negative meniscus (as is 
usually the case), and are worn for the relief of irritable and 
sensitive eyes by a hypermetropic person (as is often the case), 
it is easy to see how an aggravation of the trouble will be pro¬ 
duced, instead of an amelioration; because the hypermetropia, 
which was the cause of the irritability of the eyes, would be 
increased by the use of concave lenses, and a further cause of 
irritation be added. 

It is allowable to grind colored lenses with a weak focus 
when required, but there is a great objection to strong num¬ 
bers, where one part of the lens is much thicker than other 
parts, and where the thick part would, consequently, be very 
much darker than the thin part; this would cause the optical 
center of the lens in convex glasses to be the darkest, and in 
concave glasses to be the lightest. 

Ladies often find the protection of a veil preferable to 
colored glasses; and in the case of children, the broad brim of 
a straw hat, lined with some dark material, may answer every 
purpose. 

Sunlight, or white light, is the natural stimulus to the 
nervous elements of the retina of a healthy eye, and as such 
should be easily borne without being modified. But as there 
are many eyes that are a little weak and irritable, and as there 
are many places where the glare of the sunlight is excessive 
(as at the sea-shore or on newly-fallen snow), colored glasses 
always will be more or less used. 

Formerly green protective glasses were prescribed by 
oculists, and were in very common use. This color was no 
doubt suggested by the color of the grass and trees; but 
although reflected in this way green is very pleasant and agree¬ 
able, when transmitted through glasses it is rather irritating. 
Green glasses gradually fell into disuse, and are now scarcely 
seen at all, having "been entirely supplanted by blue—possiblv 
because the color of the sky is blue (?), but really because it is 
the complementary color to yellow, which rays of the spectrum 


THE USE AND VALUE OF GLASSES. 


149 


are thought to be most irritating to the retina; and as these 
rays abound in gas-light, blue glasses would be especially use¬ 
ful in cases of weak eyes which suffer more from use at night, 
where the harshness or intensity of the gas-light would be 
pleasantly modified by the blue, which neutralizes the irritating 
yellow rays. But while such glasses may be required in some 
few cases, there is serious objection in the large majority of 
eyes to the long-continued use of glasses of any color, which 
thus decompose the natural white light, and change or shut 
out certain colors of the spectrum, and when they are removed 
leave the retina unduly sensitive to the colors that have been 
excluded. For these reasons, the best glasses for general use, 
especially to protect against the glare of excessive sunlight, are 
those having a neutral tint, and known as neutral gray or Lon¬ 
don smoke, which do not separate the components of light, but 
which exclude each color of the solar spectrum in equal pro¬ 
portions, and thus simply diminish or soften the light. At the 
sea-shore, one sees the crop of these glasses in full bloom; and 
while on one hand it seems quite a fad for young people to 
wear them, on the other hand there are many persons whose 
eyes are sensitive to excessive glare, who find great comfort in 
their use, and who really could not enjoy the benefits of sea air 
without them; and their use in such cases cannot be objected to. 
As a rule, they should never be worn in the house, nor at any 
time when the light is not excessive and the need for them is 
not felt, as otherwise the eye would become accustomed to the 
diminished light, and would be unable to bear the natural day¬ 
light, and thus the difficulty they are intended to remove 
would only be increased. 

The simplest form of spectacles that can be used are those 
made of plain, clear glasses, for use in employments that 
expose the eye to injury from flying particles, and are recom¬ 
mended merely to protect the eyes from mechanical injury or 
excessive light. In many cases ordinary window-glass 
answers perfectly well, or if greater strength is required, plate- 
glass may be substituted. But as these latter would be uncom¬ 
fortably heavy, it has been recommended to use thin, clear 
mica instead. This idea has been made use of in the latest eye- 
protector which has been placed on the market, consisting of 


150 THE USE AND VALUE OF GLASSES. 

two oblong plates, with a hinge-like joint over the nose, and' 
fitting closely around the orbit by means of numerous bits of 
cork arranged at close intervals, and fastened around the head 
by means of a rubber band. 

But a greater amount of protection than can be given by 
this eye-protector, or by the coquille spectacles, is afforded by 
the so-called goggles, in which the glasses are built up on all 
sides with thin wire gauze in such shape as to fit snugly around 
the orbit, and secured on the head either with a rubber band 
or by regular steel temples. While these goggles shut out all 
dirt, dust, and flying particles of every nature, yet serious- 
objection is made to them by competent authorities, on the 
ground that even though the air does seem to be able to circu¬ 
late more or less freely through the gauze sides, yet they con¬ 
fine the eyes too closely, heating them and causing them to be 
half-smothered in a stuffy atmosphere saturated with their 
own vapors and secretions, and in this way aggravate and 
perpetuate inflammatory conditions of the eyes and lids, which 
they are sometimes used to relieve. 

In coquille glasses (those shaped like a watch crystal) the 
surfaces of the glass must be absolutely parallel, as otherwise 
they act as weak focusing lenses to a noticeable and even 
annoying degree. This imperfection or defect in glasses of 
this shape can be obviated only by using glasses which have 
been correctly ground, instead of moulded, as such glasses are 
usually made. 

Eye-protectors are sometimes sold under the name of 
millers’ or turners’ spectacles, with heavy frames, large eyes, 
and plain white glass, and they are not worn nearly as much 
as they should be by workmen who are exposed to flying 
chips, etc. Constant exposure to any danger causes us to* 
underrate its importance, and consequently most workmen 
take the risk of going without glasses, either because they da 
not think of the danger or because they dislike the incon¬ 
venience of wearing glasses, and, as a result, they often bear 
the scars of wounds of the cornea. On the principle of lock¬ 
ing the stable after the horse is stolen, those workmen who 
have already lost one eye in this way are easily persuaded 
thereafter to wear protecting glasses to save the other eye. 


THE USE AND VALUE OF GLASSES. 


IS* 

A prominent optical firm manufactures what they call 
Bessemer spectacles, which are a combination of colored 
lenses, for the use of persons engaged in the manufacture of 
Bessemer steel; and it is said changes in the flame can be more 
readily distinguished with their use than with any other glass. 

In the correction of any optical defect by the adjustment 
of glasses, it is of very great advantage to begin wearing them 
in youth, as the eyes then are in such a condition as to adapt 
themselves to their use much more readily than in later years. 
The eyes and glasses become, as it were, one optical instru¬ 
ment, and are almost inseparable. This fact is frequently illus¬ 
trated in the daily experience of every optician, and the 'ignor¬ 
ing of its teaching causes him no inconsiderable amount of 
trouble. Patients who somehow worry along (as they are apt 
to do) with an uncorrected optical defect until middle age, or 
until the eyes break down in the attempt to do their work 
without the assistance of the long-needed glasses, experience 
the greatest difficulty in getting their eyes accustomed to their 
use when at last they are worn, and almost exhaust their own 
patience as well as the optician’s before the glasses become 
entirely pleasant and comfortable. 


CHAPTER VIII.. 


OUTFIT REQUIRED. 


1. Optical education. 

2. Books of reference. 

3. Case of test-lenses. 

4. Complete set of test-types. 

5. Measuring-stick or metricAule. 

6. Record-book or case-book. 

7. Ophthalmoscope. 

8. Retinoscope. 

9. Prisoptometer. 

10. Ophthalmometer. 

11. Keratoscope. 

12. Phorometer. 

13. Optical bracket. 

14. Perimeter. 

OPTICAL EDUCATION. 

1. Of the value and necessity of an optical education this 
is scarcely the place to speak. That an optician who professes 
to be competent to correct the various optical defects he meets 
in his every-day experience should possess a theoretical as well 
as a practical knowledge of the whole subject is so self-evident 
as to need no lengthy argument to prove its soundness. Times 
have changed, and are changing, and people no longer buy 
their glasses of ignorant peddlers, nor allow themselves to be 
coaxed into purchasing the wonderful glasses of traveling 
optical quacks; nor are they even content to go to a store and 
pick out a pair of cheap spectacles with which they think they 
can see best for a minute or two. When their eyes ache or 
their sight blurs, they seek an educated optician in whom they 
have confidence, and expect him to be able to advise them, and 
prescribe glasses if they are found to be required. 

152 



OUTFIT REQUIRED. 


153 


The necessity for skilled opticians is apparent everywhere, 
and the time is at hand when every town and village will have 
its educated optician, and he will be a scarcely less important 
personage than its physician or dentist. And the next genera¬ 
tion will demand still more; the time is coming when the 
fitting of glasses will be placed under the same legal restric¬ 
tions as the practice of medicine and dentistry, and when no 
man will be allowed to call himself an optician, and no optician 
will be allowed to adjust glasses for defective vision, until he 
has pursued a course of study and acquired a diploma, just as 
the physician and dentist (and even the veterinary surgeon) 
is required to do before commencing the practice of his pro¬ 
fession. 

The optician of to-day is in the bloom of a business that 
is growing without limit, but only the educated opticians will 
be in a position to pluck its choicest fruits; and hence there is 
no hesitation (and probably no one will dispute this assertion) 
in placing an optical education as the first requisite in the 
outfit required by the practical optician. 

BOOKS OF REFERENCE. 

2. One could easily write a lengthy essay, and present a 
beautiful argument on the necessity of books to the profes¬ 
sional man, that would apply in great measure to the practis¬ 
ing optician, but lack of space forbids, and the truth of this 
statement will be admitted, universally, without an argument. 

One book, at least, is an absolute necessity, but it will be 
much better if the optician can have recourse to two or three, 
as in this way he gets the views not of one authority alone, but 
of different authors on the same subject. To study one book 
is necessary and important, but to follow this with a reading of 
several authors gives a breadth and depth of knowledge other¬ 
wise unattainable. Different authors treat the same subject 
differently, and each presents it from a standpoint that is new 
as well as different, and thus, by a combination of these differ¬ 
ent views and points, it is possible to gain a thorough and 
complete knowledge of the whole subject. Even if the views 
of the different authors do not always present something new, 


154 


OUTFIT REQUIRED. 


yet the reiteration of the same facts, clothed in different lan¬ 
guage, impresses them the more deeply on the mind. 

The money paid for an optical course, or for books, can 
not be regarded as spent in the ordinary acceptation of that 
word; but it represents capital, or an investment, that yields 
large returns and big percentages every day of the optician’s 
practice. 

For the further information of our readers, and in order 
to answer the many inquiries that are constantly received, we 
give below a list of the books on this subject, all of which are 
valuable and would enrich the library of the optician. At 
least two or three of them are necessary as constant com¬ 
panions, and we will mention them somewhat in the order of 
their value. 

Hartridge on Refraction is the work of an English 
oculist. The first edition appeared in 1884; other editions 
followed later. The inch system has been entirely discarded 
in this book in favor of the metrical system, and, while this 
cannot be classed as an objection, it limits the usefulness of 
the work among beginners. It does not touch the anatomy 
of the eye, but on the principles of optics and the various 
errors of refraction it is very satisfactory. Price, $1.25. 

Eye-Studies; A Series of Lessons on Vision and 
Visual Tests, by J. M. Johnston. This is a valuable book 
to the beginner. It covers the whole field of study, but does 
not go into any one subject very deeply or scientifically, which 
is rather an advantage than otherwise to those just entering 
on the study of optics and refraction. Price, $2.00. 

Anomalies of Refraction and of the Muscles of 
the Eye, by Tiffany. This is a valuable book and one that 
can be recommended. It is profusely illustrated, containing 
almost two hundred plates, embracing nearly all the instru¬ 
ments and appliances used in examining the refraction of the 
eye and the muscular equilibrium. It gives special attention 
to the diagnosis and correction of the various muscular anom¬ 
alies, and contains a very complete chapter on the ophthal¬ 
moscope. Price, $3.00. 


OUTFIT REQUIRED. 


155 


The Functional Examination of the Eye, by Clai¬ 
borne. This is a small book of only ninety-six pages, and is 
limited to the functional examination of the eye in the deter¬ 
mination of its refractive condition, and of the necessity for the 
prescribing of glasses. It does not touch on prisms or on 
muscular troubles, or the ophthalmoscope, but is confined to 
the fitting of glasses by means of the trial-case. As this is the 
means of examination in which the average optician is most 
interested, and as this book presents the subject very clearly, 
the book finds favor with the optical profession. Price, $1.00. 

Hand Book for Opticians; A Treatise on the 
Optical Trade and Its Mechanical Manipulations, by 
Bohne. This book is unique in that it lets the optician into 
the secrets of his trade, and is the result of the personal ex¬ 
perience and investigation of the author. It does not pretend 
to treat exhaustively the various refractive errors or muscular 
anomalies, for which the reader is referred to more scientific 
works. But its great claim for the practical optician is its 
dealing with the technicalities of the trade, and it contains 
many useful points not found elsewhere. Price, $2.50. 

Lectures on the Errors of Refraction and Their 
Correction with Glasses, by Valk. This is a valuable 
book on refraction, but on account of the style in which it is 
written (being the lectures delivered by the author to the 
physicians who attended the section on diseases of the eye at 
the New York Post-Graduate School), it is best understood 
and appreciated by the more advanced and experienced opti¬ 
cians. Special attention is given to retinoscopy and astig¬ 
matism, and the diagnosis of refraction by the ophthalmo¬ 
scope. Price, $3.00. 

Refraction of the Eye, by Morton. A small book, 
but a useful one. Price, $1.23. 

Functional Nervous Diseases, with a Supplement 
on the Anomalies of Refraction and Accommodation 
of the Eye and of the Ocular Muscles, by Stevens. 
While the whole book treats of nervous disturbances as caused 
by difficulties attending the functions of accommodation and 


OUTFIT REQUIRED. 


156 

of adjusting the eyes in the act of vision, yet it is the supple¬ 
ment that especially interests the optician, wherein consider¬ 
able space is given to the treatment of muscular anomalies 
and the cure of nervous complaints by their removal. Price, 
$2.50. 

Spectacles and Eye-Glasses, Their Forms, Mount¬ 
ing and Proper Adjustment, by Phillips. This little book 
is the outgrowth of the instruction on the subject of prescrib¬ 
ing spectacle frames, which has been given to the eye classes 
at the Philadelphia Polyclinic. It is intended to supplement 
studies in refraction, and to give the student knowledge of the 
correct placing of the glasses before the eyes. Price, $1.00. 

As treating on special branches of the subject we would 
mention the following: 

An Atlas of Ophthalmoscopy, with an Introduc¬ 
tion to the Use of the Ophthalmoscope, by Haab. Price, 
$ 3 - 5 °- 

How to Use the Ophthalmoscope, being elementary 
instructions in ophthalmoscopy. Arranged for the use of 
students, by Browne. Price, $1.00. 

Hartridge on the Ophthalmoscope. Price, $1.50. 

The Clinical Use of Prisms, and the Decentering 
of Lenses, by Maddox. Price, $1.50. 

New Truths in Ophthalmology, by Savage. Atten¬ 
tion is drawn to the action and functions of the oblique mus¬ 
cles, concerning which the author presents what he calls new 
and valuable truths. The book does not treat on refraction, 
but is confined to the consideration of muscular troubles. 
Price, $2.00. 

Skiascopy, and Its Practical Application to the 
Study of Refraction, by Jackson. Price, $1.00. 

Color-Vision and Color-Blindness, by Jennings. 
Price, $1.00. 

As popular works on the subject, we would mention: 

Eyesight and How to Care for It, by Harlan. 
Price, 50 cents. 


OUTFIT REQUIRED. 


157 


Eyesight, Good and Bad. A Treatise on the Exercise 
and Preservation of Vision, by Carter. Price, $1.25. 

The Eye and Its Care, by Allport. Price, $1.00. 
This list is not intended to be exhaustive, but includes those 
books which the writer has found most useful in his work of 
instructing opticians. 

These books will be furnished by The Keystone, Nine¬ 
teenth and Brown streets, Philadelphia, Pa., at the prices 
named, subject to changes by the publishers. 

CASE OF TEST-LENSES. 

3. No optician in these days can expect to make a success 
of the business of fitting glasses without a test-case of trial- 
lenses. Every dealer who has many eyes to test, and who 
wishes to do it carefully and thoroughly (and it should never 
be done in any other way), soon finds out that it is not con¬ 
venient or satisfactory to have to depend on the spectacles and 
eye-glasses that are kept in stock and for sale, and he finds 
himself compelled, sooner or later, to invest in a test-set. The 



The Test-case. 


public, generally, are awakening to a realization of the im¬ 
portance of these matters, and they will certainly prefer to 
patronize the optician who is best prepared to examine their 
eyes, and they will not be slow to see that this can be better 












158 


OUTFIT REQUIRED. 


done by the use of test-lenses than by a miscellaneous assort¬ 
ment of spectacles of various numbers strewed around on the 
counter in confused heaps. Then, again, the testing of each 
eye separately, which a thorough examination necessitates, 
cannot, of course, be done by spectacles of similar glasses 
taken from stock, but can only be done by the use of a set of 
test-lenses and a trial-frame in which one eye can be excluded 
at a time. 

It can be laid down as an inflexible rule, to which none 
can take exception, that after the acquisition of a complete 
knowledge of the whole subject of optics and fitting glasses, 
the next most important item is the possession of a trial-case, 
and the assertion is none too strong when it is said that no 
dealer is worthy of the name of optician whose outfit does not 
include a test-set. 

Acknowledging that every optician must have a test-case, 
the size and cost of the one he should procure will probably 
depend on the size of the town in which he does business and 
the amount of attention he expects to give to this department 
of his store. But the amount paid for it is money well invested, 
and the larger and better the test-case the more it will add to 
its owner’s reputation as a competent optician, and the better 
it will enable him to fit his cases, and, as a consequence, the 
more cases he will have to fit. 

One of the advantages of the possession of a set of test- 
lenses that might be mentioned, is that the optician is enabled 
to ascertain the focus and strength of any simple or compound 
lens that comes into his hands, by neutralization, which sub¬ 
ject has been elaborated at another place on these pages. 

A complete trial-case is composed of sample lenses of the 
various kinds used to correct optical defects (sphericals, cylin¬ 
drical, prisms), which are placed in the trial-frame before the 
patient’s eye, and are readily changed without taking the 
frame from the patient’s face; the examination in this case 
being what is called, subjective—that is, dependent on the 
answers given by the patient himself. When the correct lenses 
are thus determined by trial, the optician, from his stock, can 
quickly furnish similar lenses set in a suitable frame, if they 
are spherical; or can grind them if cylindrical or prismatic. 


OUTFIT REQUIRED. 


159 


In contradistinction to the subjective examination as 
above, may be mentioned an examination by the ophthalmo¬ 
scope, which is entirely objective—that is, the optician selects 
the proper lens unaided by the patient’s answers, which 
method will be described in the chapter on the ophthalmo¬ 
scope. 

The trial-case should contain thirty-two pairs of convex 
spherical lenses, and thirty-two pairs of concave spherical 
lenses, of the following numbers: 


0.25 D. 

2.25 D. 

5.00 D. 

11.00 D. 

0.50 

2.50 

5.50 

12.00 

0.75 

2-75 

6.'oo 

13-00 

1.00 

3.00 

6.50 

14.00 

1.25 

3-25 

7.00 

15.00 

1.50 

3 .* 5 o 

8.00 

16.00 

i -75 

4.00 

9.00 

18.00 

2.00 

4-50 

10.00 

20.00 

se should be 

in pairs , and both convex 

and concave. 


In addition to the spherical lenses, there should be eighteen 
convex and eighteen concave cylindrical lenses, numbered 
from 0.25 D. to 6.00 D. The cylinders do not run above this, 
as it is very rare that a stronger one is required. These may 
be single, but it is much more satisfactory to have them in 
pairs. 


The case should also contain about ten prismatic lenses 
(plane prisms, without any focus), numbering from one degree 
to twenty degrees. 

Besides these, the case contains usually a few colored 
lenses—light blue and dark blue, and light red and dark red; 
also a plane glass, a ground glass, and a glass with one-half 
the circle clear and plane and the other half ground or frosted. 
Also several metal disks, one of which is solid, to be used to 


exclude one eye while the other is being tested. Another has 
a minute perforation in its center, called the “pin-hole” disk, 
which is to be used in any case of imperfect vision to deter¬ 
mine whether the impaired vision can be corrected by glasses 
or whether it is due to organic disease and beyond the reach of 
glasses. If the pin-hole disk improves vision, then the sight 
can be restored to an equal degree by properly adjusted 


i6o 


OUTFIT REQUIRED. 


glasses. If the pin-hole disk causes no improvement in the 
appearance of the letters on the test-card hanging in a good 
light twenty feet away, then glasses will be of no avail, and it 
would only be a waste of money and time to try to fit them. 

A third metal disk is called the “stenopaic slit/' in which 
there is an adjustable bar which can be moved to widen or 
narrow the opening, as may be desired. This stenopaic slit is 
used in the detection and correction of astigmatism. The slit 
is placed in the trial-frame and rotated to the meridian of best 
vision, which is corrected by a spherical lens; it is then rotated 
at right angles, which meridian is also corrected by a spherical 
lens. The refraction of the two principal meridians is thus 
known, and the proper correcting cylindrical lens can be cal¬ 
culated therefrom. 

There are some trial-cases sold in which the lenses are 
unmounted, and they are only mentioned to be condemned; 
their single advantage is that they can be sold for less money, 
but they cannot be called cheaper. The liability to soil them 
with the fingers, the difficulty to handle them easily and to 
place them in the trial-frame and remove them quickly, and 
the danger of chipping and breaking, as well as their incom¬ 
plete appearance, make of them a very undesirable case. When 
the lenses are furnished mounted, they are burnished into 
metal rings—white metal being used for convex lenses and 
yellow metal for concave lenses, with the number of the lens 
stamped on the handle. In the cases of unmounted lenses the 
number is scratched on the lens itself, preceded by the plus (+) 
or minus (—) sign to indicate its refraction. Likewise prisms 
are marked (on the handle of ring) with the number of the 
prism in degrees. In the same manner cylindrical lenses are 
marked with the number of the lens, and, in addition, are 
marked to show the direction of the axis; besides which, in a 
great many cases, they are frosted or made opaque on either 
side of these marks and along the edges of the lens, this being 
done to show more quickly at a glance the direction of the 
axis, as the side sections of ground glass have straight borders 
parallel to the axis. The size of test-lenses (that is, the 
diameter from edge to edge of ring) is about one and one-half 
inches. 


OUTFIT REQUIRED. 


161 


In some of the later and better test-cases the rings are 
different from those above described; instead of the lenses 
being burnished into the rings, according to the old method, 
these rings can be opened and the lens placed in the groove 
and firmly secured by means of a small screw, which fastens 
them in the same manner as spectacle lenses are secured in 
spectacle frames. This is considered an advantage, as the 
lenses can be removed or replaced in case of breakage, or 
error in lens, without injury to either lens or ring, which can¬ 
not be so readily done when they are burnished in. These 
rings are made of steel and nickel-plated, and both convex and 
concave of the same color, but they are distinguished by the 
plus or minus mark being cut out of the handle. These 
nickel-plated rings will perhaps wear longer than the ordinary 
rings, of which some are gilt and some silvered. 

Sometimes the cylindrical lenses in the trial-frames are 
furnished without handles, so that the axis can be the more 
readily rotated completely around the circle to any desired 
degree; but those which have the handle are preferable, 
because they can be more easily manipulated, and usually no 
difficulty is experienced in rotating them to the proper posi¬ 
tion, on account of the way they are set in the ring and the 
construction of the trial-frames that are used. 

A trial-case of test-lenses is the greatest necessity of the 
optician, but the high price at which such cases have been sold 
(until recently, when they can be purchased much more rea¬ 
sonably) has acted as an obstacle in the way of the optician 
giving the attention to his business which it deserves. The 
effort on the part of manufacturers has been to furnish trial- 
cases as inexpensive as possible, in order that they might be 
within the reach of a much larger number of opticians, and as 
a consequence a great variety of different cases have been 
advertised and placed on the market; but as the effort to 
cheapen the cases goes on, the efficiency of the case rapidly 
declines in proportion, until they are hardly of enough prac¬ 
tical value to justify the outlay for them. A great many of the 
numbers are left out, and when one of these numbers is wanted 
it can only be obtained by combining two or three lenses 
together in the clip; and when using the dioptric system 


162 


OUTFIT REQUIRED. 


combinations can be very easily made without any knowledge 
of mathematics whatever. 

The most important instrument contained in the test-case 
is the trial-frame, the, use of which is to grasp the test- 
lenses firmly and hold them before the eye, and at the same 
time allow them to be readily changed. The simpler forms 
are not unlike heavy spectacle frames, with semi-circular or 
half-eyes, rounded to hold the circular lenses of the trial-set, 
and usually with straight temples, although hook temples may 
be substituted. The “eye” of the trial-frame (or, more liter¬ 
ally, the half-eye) is made up of two or three grooves, in which 
as many lenses can rest; or, in place of the grooves, may be 
fitted with three hooks, at equidistant intervals, on both inside 
and outside of frame, which answer the same purpose of hold¬ 
ing the lenses before the eyes. In frames like these, only 
spherical lenses are intended to be used; while for cylindrical 
lenses the face of the frame must be graduated around the 
semi-circle in degrees, and in this the axis of the lens can be 
turned to any degree that is found necessary. 

The rotation of the cylindrical lens is accomplished in 
many trial-frames by the movement of the lens, while in some 
of the more elaborate frames the outer lens-holder is movable, 
so that it can be rotated, and thereby the axis of the lens can 
be brought to any desired angle without any turning of the 
lens itself. The inner, or posterior, lens-holder is for spherical 
lenses, and remains stationary, not being affected by the revo¬ 
lution of the holder carrying the cylindrical lenses. 

The graduations on cylindrical trial-frames are always the 
same—commencing at zero, at the patient’s left, and proceed¬ 
ing upward to fifteen degrees, thirty degrees, and up to ninety 
degrees, which is exactly vertical, and down to one hundred 
and eighty degrees, which is exactly horizontal; and then 
commencing at this point, which is the patient’s right, and 
going down to ninety degrees, which is vertical, and proceed¬ 
ing up to one hundred and eighty degrees, which is horizontal. 
It will be observed that the vertical meridian is always at 
ninety degrees, while the horizontal meridian may be either 
zero or one hundred and eighty degrees (in prescriptions it is 
usually written i8o°, although sometimes the capital letter H 


OUTFIT REQUIRED. % 163 

is used to indicate the horizontal meridian); and it should be 
further noted that these figures refer to the frame as it sets on 
the patient’s face and as seen by the optician standing in front 
of it. This is sometimes a difficult matter to understand 
(where to commence and where to end counting), and the fol¬ 
lowing diagram is introduced in the hope of making it 
plainer: 



Much experience with opticians leads to the belief that 
many of them do not have clear and definite knowledge of this 
matter; and it sometimes proves itself a bugbear even to those 
otherwise well informed. For instance, a number of years ago, 
one of the medical students attending the clinics at the Wills 
Eye Hospital (the most noted hospital of its kind in Phila¬ 
delphia) asked the surgeon in charge, who was an eminent 
oculist, where the numbering commenced of the degrees used 
to denote the position of the axis of cylindrical lenses. The 
oculist replied on the right, then corrected himself, and said 
on the left, and seemed to be confused; and was really unable 
to give a definite reply to the question until he had called 
for a trial-frame and given the matter several moments’ con¬ 
sideration. Of course, the ophthalmic surgeon of a large 
hospital gives more attention to, and takes more interest in, 
cases of disease—such as the various inflammations to which 


164 


OUTFIT REQUIRED. 


the eye is liable, and which require careful treatment—or 
cataract and cross-eyes, which call for the use of the knife, 
and which all afford an opportunity for the exhibition of the 
surgeon’s skill and for the accomplishment of brilliant results; 
while refraction cases, which are tedious, and try the physi¬ 
cian’s patience, are left for the younger doctors to correct, and 
on whom they must gain their experience. But still, in spite 
of this, one would think an oculist of age and experience 
would have been able to answer this question without a 
moment’s thought; and yet this little incident serves to show 
that this question may prove a source of confusion as well to 
the educated physician as to the less pretentious optician. 


LEFT. RIGHT. 



The principal source of confusion arises from the differ¬ 
ence between the notation of the degrees as it appears on the 
front of the trial-frame and on the oculists’ prescription blanks ; 
and illustrations of both have been introduced here, so that 
the comparison can be readily made between them, and the 
source of error be removed. 

Repeating, then, what was said before, the notation always- 
commences at zero, at the patient’s left and proceeds upward to 
ninety degrees, and downward on the opposite side, or on the 
patient’s right, to one hundred and eighty degrees, and thus 
completes the semi-circle. If the graduation is on the lower 
segment of the circle, it commences at the patient’s right, and 
goes down to ninety degrees, and up to one hundred and 
eighty degrees at the patient’s left. With this fact firmly fixed 
in the mind, a comparison between the two diagrams given 
above will show that there is really no conflict or difference 
between them, but that they are both graduated exactly as this 



OUTFIT REQUIRED. 


165 


rule directs. In the first illustration, the numbers are seen on 
the face of the trial-frame as they appear to the optician stand¬ 
ing in front; while in the second diagram, the degrees are 
numbered as they would appear to the eyes of the wearer look¬ 
ing at the posterior surface of the glasses, if seen by him, and 
particularly as they would appear to the manufacturing opti¬ 
cian as he grinds the lenses into the frame, because he works 
from the posterior surface. A thorough understanding of 
this subject by the optician now will save him much annoy¬ 
ance and confusion in the future. 

A great many styles and varieties of trial-frames have been 
placed on the market by the various manufacturing optical 
houses, each of which is claimed to be the best. The points to 
he desired in a trial-frame, in order that it may meet every 
requirement, are grooves for two lenses, the front of which is 
to be graduated in degrees for cylinders; movable lens- 
holders, so that they may be adjusted to the distance between 
the eyes to be tested; a movable nose-piece that may be ad¬ 
justed to hold the lenses at the proper height before the eyes; 
the whole to be made of some material that will not be too 
heavy. 


COMPLETE SET OF TEST-TYPES. 

4. A complete set of test-types is the next requisite in 
the outfit needed by the optician, which will include test-letters 
for distant vision, test-types for near vision, and astigmatic 
cards of radiating lines and letters. 

The card of test-letters used for testing distant vision is 
made up of block-letters, the strokes and finite of which are 
solid black, with parallel edges, their width being exactly one- 
fifth the height of the letter, and the width and height being 
the same, and printed on heavy card-board in a manner con¬ 
venient for hanging on the wall. A practical point is to have 
two or three of these cards, with the letters arranged differ¬ 
ently in order, as a patient whose eyes are examined more 
than once soon becomes familiar with the letters on one card, 
•and might be able to repeat them from memory, and thus un¬ 
knowingly mislead the optician, who could not know whether 
the letters were repeated from sight or from memory. 


OUTFIT REQUIRED. 


166 


In order that the image formed on the retina may be of 
sufficient size to excite perception, the object which produces 
the image must be seen under a certain visual angle. Now the 
smallest retinal image which can be perceived at the yellow 
spot of the retina corresponds to a visual angle of one minute 
(i')> so that if two points were separated by an interval of less 
than one minute, the eye would be incapable of perceiving the 
separation between them, and they would produce upon the 
eye the effect of but a single point. The visual angle (it may 
be remarked) is the angle included between two lines drawn 
from the top and bottom of the object which converge to and 
cross each other at the nodal point of the eye, which is situated 
just back of the crystalline lens. 

These principles were made use of in the construction of 
the test-letters which are used for determining the acuteness of 

s 


ws 

Hi 


2f5i CP sffi ssa 

m m Es m 

za. 

A F EQR F D Z 20 

sr. 

NPRTVZBDFHKO 15 

an. 

SUYACEGLHPRT12 





VZBDFHK0ST77A10 

Test letters for Distant Vision. 













OUTFIT REQUIRED. 


167 


vision, the letters being drawn so that each limb and subdivi¬ 
sion and space would subtend an angle of one minute at the 
nodal point, while the height and width of the letter would be 
five times the width of the limbs, causing the letters as a whole 
to subtend an angle of five minutes at the same nodal point. 

In other words (referring to the visibility of the test- 
letters used for distant vision), the width of the lines of the 
letter forms on the retina two points, just barely far enough 
separated to be distinguishable as two points, while the five 
lines or spaces which make up the size of each letter will then 
form an image on the retina, which, in the majority of cases, 
will be the smallest which the normal retina can appreciate. 

The test-letters of Snellen, which are constructed on these 
principles, are those in most common use. They are drawn in 
the proportions mentioned, and in different sizes, each of 
which is marked with a number, which indicates the distance 
in feet at which the letter should be distinctly seen, and at 
which the height and width of the letter will be seen under a 
visual angle of five minutes, and the limbs of the letter at an 
angle of one minute. The top row of letters should be dis¬ 
tinctly legible at two hundred feet; the next at seventy feet; 
the next at forty feet; and so on to the smallest, the letters of 
which can be named at ten feet; and it should be remembered 
that the angle produced by the large letters at two hundred 
feet and that produced by each of the letters at their respective 
distances is exactly the same as that caused by the smallest 
letters at ten feet. 

It is maintained by Snellen that in order to be able to dis¬ 
tinguish one letter from another the eye must be able to notice 
the width of the lines which make the letter, and also the 
spaces between the lines; and as it is desired to make this test 
as delicate as possible, the width of the lines and the spaces be¬ 
tween them are drawn so as to correspond to a visual angle of 
one minute, which is the smallest space which the retina is 
capable of perceiving. This applies to a great many of the 
letters, as, for instance, to differentiate between C and G and 0, 
where the eye must be able to> distinguish the white space 
which interrupts the circle in C and G, and must also be able to 
notice the addition to G which C lacks. The same is true of 


OUTFIT REQUIRED. 


168 

E and F, where the eye must be able to make out the additional 
line at the bottom of the E, and likewise between B and H, 
which appear similar in shape at a distance, and require to be 
really seen before they can be differentiated. 

The test-card for distant vision should be hung on the wall 
in such a position as to be well illuminated by a good light, and 
far enough away from the patient to avoid the necessity for 
any effort of accommodation. Only parallel rays are focused 
on the retina of the emmetropic eye without the aid of the 
accommodation; and only those rays are absolutely parallel 
which proceed from objects at infinite distance. But for the 
practical purposes of the optician in testing distant vision, 
twenty feet has been by common consent agreed upon as the 
standard distance at which the best results can be attained in 
such examinations; and every optician is advised and urged 
to test his cases at this distance, if the size of his store or office 
will permit. Rays of light proceeding from an object at twenty 
feet may be assumed to be parallel, and are so near parallel as 
not to disturb the calculation. Oftentimes this distance cannot 
be secured, and the examination must be made at fifteen feet, 
or twelve feet, and sometimes even at ten feet; and while satis¬ 
factory results may be attained at these distances, it should 
always be remembered that with every approach to the eye 
there is some effort of the accommodation called for, however 
slight. 

Snellen’s test-letters are those in most common use, and 
they seem to be suited for all practical purposes, and to yield 
satisfactory results. At the same time, it is well to know that 
there are other test-types in use in different countries, but the 
only ones which seem to have any value comparable with 
those of Snellen are those of Green and Monoyer; which latter, 
however, we will not take the space to describe, as Snellen’s 
will be the only ones which our readers will be likely to use. 

The test-types for near vision are usually selections of read¬ 
ing-matter of different sizes, each of which is marked with the 
distance at which it should be seen by a normal eye with good 
sight. Smaller block-letters, graded on the same scale as 
Snellen’s distant letters, are sometimes used, but those most 
commonly preferred are Jaeger’s, because his letters are of the 


OUTFIT REQUIRED. 


169 


ordinary shapes, although they have the disadvantage that they 
are not arranged on any scientific plan, but are simply printers’ 
types of various sizes. 

The value of selections of reading-matter as a test is very 
much impaired by the fact that persons who are accustomed to 
reading are able to guess at the majority of words by their 
general appearance and their relation to neighboring words, 
while illiterate and uneducated persons must decipher the 
letters one by one, which places the latter in a more unfavor¬ 
able position than the former, and therefore vitiates, to some 
extent, the value of the test as not being a certain proof of 
visual acuteness. This is sometimes remedied by the use of 
words intentionally mispelled (Josh Billings’ style); but for an 
accurate test, isolated letters should be used, constructed on 
the same principle as the larger test-types. 

Occasionally the optician meets with a customer who can¬ 
not read, and sometimes he is called upon to examine the eyes 
of young children who do not know their letters. Examina¬ 
tions of such persons are always more or less unsatisfactory, 
and the desire should be to make t ; hem as nearly accurate as 
possible. For use in such cases, cards of numbers are printed 
in various and increasing sizes; but what is more preferable 
(and is even necessary for those who cannot tell the numbers) 
is a card of figures with projecting arms like a capital letter E. 
These are printed in the same sizes as the letters on the ordi¬ 
nary test-card, and with the arms pointing up and down, to the 
right and to the left. The test of the patients’ ability to see 
these figures is determined by their ability to tell in which 
direction the arms point. 

The optician’s selection of test-types will embrace several 
cards for the determination of the existence or non-existence 
of astigmatism. Those in most common use are the radiating 
lines, somewhat similar to the face of a clock (to which it is 
oftentimes compared), and Dr. Pray’s series of astigmatic 
letters, which are made of black lines and white spaces, the 
lines and spaces running at the same angle in each letter, and 
■every letter representing a different angle. The full card is 
composed of twelve letters, which are drawn at the following 


170 


OUTFIT REQUIRED. 


angles: 15 0 , 30°, 45 0 , 6o°, 75 0 , 90°, 105°, 120°, 135 0 , 150°, 
165°, and 180 0 . 



Attention should be given to the numbering of the angles,, 
and comparison made between the above figures and those on 
a preceding page representing the graduated front of trial- 
frame and the graduations on the prescription blanks. In this 



Dr. Pray’s Astigmatic Letters. 


way the optician will have clearly fixed in his mind the method 
of numbering the degrees on the astigmatic semi-circle, which 
is (and it is repeated again, that it may become indelibly fixed 
in the memory) to commence at the patient’s left and complete 
the semi-circle, or the 180°, at the patient’s right. 

MEASURING-STICK, OR METRIC-RULE. 

5. This is used to measure the range of accommodation 
and to determine the near-point and far-point, and in the ab¬ 
sence of a short rule may be used to measure the pupillary 











OUTFIT REQUIRED. 


171 

distance. An ordinary yardstick will answer the purpose, but 
the optician is advised to procure, as more preferable, a metric 
measure, which is exactly one meter long, and is marked on 
one side in centimeters and millimeters, and on the other side 
in inches and fractions of inches. 


RECORD-BOOK, OR CASE-BOOK. 

6. Method and system are of advantage in the conduct of 
any business, but they are especially invaluable to the optician, 
who must keep a methodical and systematic record of his 
cases if he wishes to do business with pleasure and profit to 
himself as well as to his customers. No man who does business 
in a slidshod, careless way can succeed, and he doesn’t even 
merit success; while the careful, systematic man achieves suc¬ 
cess because he deserves it. 

No optician should sell a pair of glasses without making 
a record of the sale. It will take but a moment’s time to write 
down the date, the name of the customer, and the number of 
the glasses sold, with, perhaps, the style of frame and the price 
paid. But some one will say that it does not pay to go to so 
much trouble with a pair of twenty-five-cent glasses, and with 
this statement I cannot disagree. But here is a broader and 
stronger statement, which undoubtedly will strike a responsive 
chord in every optician’s breast— don’t sell such cheap glasses 
that you cannot afford to take the time to fit them and make a 
record of them . 

When a customer asks for a pair of cheap glasses, a hand¬ 
ful of various numbers is laid on the counter, and he is told to 
help himself; he picks out a pair with which he thinks he can 
see, and pays his money, just as if he was buying a shirt collar, 
and without the advice or responsibility of the optician. 
This is certainly wrong—it is hurtful for both optician and 
customer; the glasses may damage the patient’s eyes and the 
optician’s reputation at the same time. This is not the place 
to make a more elaborate argument in this direction, as the 
matter is only mentioned incidentally in support of the state¬ 
ment that every optician should keep a record of the glasses 
he has fitted, but I have a very positive opinion on this subject 


172 


OUTFIT REQUIRED. 


that it does not pay the optician to keep and sell cheap glasses; 
it does not pay either optician or patient; but if the latter will 
insist on risking his eyes with such trash, then let the optician 
wash his hands of responsibility and permit his customer to 
go elsewhere to buy them. 

The case-book or record-book, therefore, is considered as 
a necessary part of the outfit of every optician. 

These record-books may be divided into two classes— 
those which are designed to keep a simple record of the glasses 
sold, and those which are intended to preserve a complete 
record of the examination of the vision as well as of the glasses 
prescribed. 

The first or simplest class of record-books can be used as 
a register for glasses sold for simple presbyopia, and for all 
simple glasses that are fitted without any very extended or 
elaborate examination. The book can be ruled or spaced for 
the following headings: Date, Name of Customer, Number of 
Glasses, Style of Frame, Price. Any ordinary blank book can 
be purchased and ruled as above by the optician himself, or the 
books can be ordered ready-ruled. An index should accom¬ 
pany the book, and if the name is entered in the index at once 
it consumes but little time and makes but little trouble; while 
if the names are allowed to accumulate, the indexing becomes 
a tedious task and is apt to be neglected. This destroys the 
value of the record-book, as its usefulness depends on the 
ability to find a given name and find it quickly, which can 
only be done by the aid of an index. The optician should 
take pride in keeping this book neatly and carefully, and it 
will be a source of satisfaction to him as his optical business 
grows and his customers return, and he desires to look up the 
glasses previously sold to them. 

The second class of record-books should be much more 
complete and elaborate, so that they will not include the 
simple record as above, but will contain a complete record of 
acuteness of vision, range of accommodation, and all the 
points revealed by a careful examination of the eyes. Such a 
book should embrace the following headings: 


OUTFIT REQUIRED. 


m 


No. 

Date. 

Name. 

Age. 

Residence. 

Occupation. 

History of Case. 

Present Condition. 

Symptoms Complained of. 

Ever Worn Glasses? 

If So, What Number and How 
Long? 

Any Other Remarks. 

Vision. R. E. L. E. 

Refraction. R. E. L. E. 

Accommodation. N. P. F. P. 

With- N. P. F. P. 

Insufficiency of Muscles. 
Astigmatism. 


Distance. R. E. 

L. E. 

Reading. R. E. 

“ L. E. 

Ophthalmoscopic Appearance.. 
R. E. 

L. E. 

Diagnosis. 

Suggestions. 

Report of Case. 

Frame. 

Pupillary Distance. 

Depth of Bridge. 

Inclination of Bridge. 
Distance. R. E. 

“ L. E. 

Constant. R. E. 

“ L. E. 

Reading. R. E. 

“ L. E. 


Such a book also serves to suggest the different steps that 
should be followed in making a complete examination of the 
vision, and is really indispensable to the thorough optician. 
A record-book, such as is described above, which was spe¬ 
cially compiled for the use of the optician, and is at the same 
time complete and simple of arrangement, can be had at 
The Keystone office for one dollar. This book has an index,, 
an indispensable portion of every serviceable record-book. 
Some of the more systematic opticians keep a separate index 
book, in which they enter the names of all customers from 
both the above-mentioned classes of record-books. 

OPHTHALMOSCOPE. 

7. The ophthalmoscope, which was formerly looked upon 
as an instrument for use by the medical faculty alone, has now 
a place in the outfit of every well-equipped optician. It is not 
unusual, however, to find among the less experienced opticians 
an exaggerated and erroneous impression as to the purposes 
for which an ophthalmoscope should be used, and the in¬ 
formation that can be derived from its employment. Some 
of these opticians entertain such an exalted opinion of this in* 



*74 


OUTFIT REQUIRED. 


strument, that it appears as if they must be under the impres¬ 
sion that when they look into an 
eye with the ophthalmoscope, 
they will be able to see stamped 
in the eye, in plain figures, the 
number of the glass required to 
correct that particular case. In 
fact the ophthalmoscope is 
looked on by some optical stu¬ 
dents and opticians as a magical 
and mystical instrument with a 
mysterious something about it 
which makes it difficult to under¬ 
stand its use, but which, when 
comprehended, affords them an 
infallible method for fitting the 
most difficult cases, to the exclu¬ 
sion of every other means. 

Riper experience brushes 
away these pleasing delusions 
and strips the ophthalmoscope 
of much of its importance in 
the hands of opticians, who are 
interested only in correcting 
optical defects. In the case 
of physicians it is very different; 
as, in the treatment of diseases 
of the interior of the organs of 
vision, the ophthalmoscope be¬ 
comes a necessity, to note the 
.•advance of disease and watch the effect of treatment. Besides 
which, the value and usefulness of this instrument are in direct 
proportion to the skill of the operator; hence its decreased 
value in the hands of the optician, who does not have the op¬ 
portunities to acquire the necessary expertness in its use as 
■does the physician. 

In using the ophthalmoscope to diagnose errors of refrac¬ 
tion, the physician or optician examines the fundus of his 
patient’s eye. If his own eye and the patient’s eye are both 

































































OUTFIT REQUIRED. 


175 


normal, a perfect image of the optic disk and the retinal vessels 
can be obtained; but if any ametropia exists in either physi¬ 
cian’s or patient’s eye, the ophthalmoscopic picture will be to 
that extent marred, and the lens required to afford a perfect 
view of the fundus will be the measure of the ametropia. In 
these cases it is absolutely necessary that the observer know 
his own refraction, and that if any ametropia be present it be 
corrected by the proper lenses, after which he is in a position 
to examine the fundus of his patient’s eye and ascertain the 
number of the neutralizing lens required to afford a perfect 
image of it. Under the most favorable circumstances and with 
the greatest dexterity on the part of the observer, this affords 
but an approximate correction; and hence it can be laid down 
as a broad, general rule, that no error of refraction can be accu¬ 
rately and satisfactorily measured or corrected by the ophthalmo¬ 
scope alone; but the use of test-letters and test-lenses is by far 
the most reliable (as it is the most used) method of measuring 
and correcting these refractive errors, reserving the ophthal¬ 
moscope to verify the results thus obtained. 

In this connection the use of the retinoscope may be men¬ 
tioned, or in other words, “the shadow test.” This is a pro¬ 
cedure only brought into use of late years; and while it is de¬ 
scribed as popular and effective, yet it really does not seem to 
be coming into common use very fast. The method is learned 
and executed only with considerable difficulty, and depends on 
the movements of the fundus reflexes when reflected light is 
thrown into the eye and the mirror is rotated about its vertical 
or horizontal meridian, these movements being different for 
the different refractive errors. These movements are depend¬ 
ent on the condition of the emergent rays—those from an em¬ 
metropic eye being parallel, from a hypermetropic eye diverg¬ 
ent, and from a myopic eye convergent. The use of the oph¬ 
thalmoscope and retinoscope in the diagnosis and correction 
of refractive errors will be fully and thoroughly explained and 
demonstrated later on, when we come to the proper chapter. 

RETINOSCOPE. 

8. The more complete trial-cases usually contain one of 
these instruments, the use of which is at present attracting the 


176 OUTFIT REQUIRED. 

attention and engaging the interest of all progressive opti¬ 
cians. 

Retinoscopy, or keratoscopy, or pupilloscopy, or skia¬ 
scopy, or the shadow-test, as it has been variously termed, is 
a valuable auxiliary method for de¬ 
terminating the refraction of the eye, 
and it is one which the optician can¬ 
not afford to negect. It is espe¬ 
cially useful in the examination of 
children or of uneducated persons. 

This method of determining the 
refraction depends upon the direction 
of the movements, in the pupil, of the 
shadows cast by the fundus reflexes, 
when light is reflected into the eye by 
the retinoscopic mirror, which is then 
rotated vertically or horizontally. 

These movements are different 
in the several refractive errors, being 
dependent on the condition of the 
emergent rays, those from an emme¬ 
tropic eye being parallel, from an 
hypermetropic eye divergent, and 
from a myopic eye convergent. 

The room should be darkened and the patient placed 
directly under the light. The optician is seated at a distance 
of one meter, and reflects the light directly into the eye, which 
produces the red reflex in the pupil. The mirror is then 
rotated and a movement of the red reflex in the pupil is 
caused, which is followed by an area of shadow. 

If the mirror of the retinoscope be plain (which is prefer¬ 
able), the movement of the shadow will be in the same direc¬ 
tion as the tilting of the mirror, if the case be one of emme- 
tropi'a or hypermetropia. While the shadow produced will 
travel in the opposite direction from the way the mirror is 
tilted, if the case be one of myopia. In the emmetropic eye 
the shadow is quite dim, and a weak lens is sufficient to re¬ 
verse its direction. In the hypermetropic eye the shadow is 
more distinct. 






OUTFIT REQUIRED. 


177 


The degree of the defect is determined by that concave 
glass in myopia, and that convex glass in hypermetropia, that 
dispels the shadow or slightly turns it in the other direction 
from which it moves naturally. 



THE PRISOPTOMETER. 

9. The prisoptometer is an excellent instrument for the 
detection and correction of the various refractive errors. It 
consists essentially of a double 
prism, which can be revolved 
from o to 180 0 . As the patient 
looks through the circular 
opening in the center of the 
instrument, at the white ob¬ 
ject circle, he will see two 
objects, and the relation they 
bear to each other will indi¬ 
cate whether the case be em¬ 
metropic or ametropic, and 
also point out the nature of 
the ametropia. 

If the white circles are 
simply in contact, the case is 
one of emmetropia. If the 
circles overlap each other, the 
case is one of myopia, and the 
concave lens that separates 
them to mere contact will be 
the measure of the myopia. 

If the circles are separated 
from each other, the case is 
one of hypermetropia, and the 
convex lens that restores them 
to mere contact will indicate 
the amount of the hyperme¬ 
tropia. 

The revolving of the 
prism will cause a revolution 
of the circles, and if they maintain the same relative position 





i 7 8 


OUTFIT REQUIRED. 


all the way around the eye is spherical in shape, or, in other 
words, there is no astigmatism; whereas, if astigmatism is 
present, the circles will separate or overlap at some one point, 
thus indicating either hypermetropic or myopic astigmatism, 
which is then corrected by a convex or a concave cylinder, 
with its axis at right angles to this point. 

OPHTHALMOMETER. 

io. This instrument is invaluable in measuring the curva¬ 
tures of the cornea, and in estimating the degree and kind of 
astigmatism. Its scope is limited to this defect, and it is of no 



use in determining the refraction; but in spite of this, the in¬ 
formation which it conveys is so direct and positive, that it is 
conceded to be an instrument of such inestimable value as to 









OUTFIT REQUIRED. I79 

justify the progressive optician in making so expensive an 
addition to his outfit. 

Of late years there has been considerable discussion as to 
the value of the ophthalmometer, largely because there has not 
been the proper appreciation of the purport of the instrument, 
as well as its limitations; and the statement cannot be made too 
emphatic that its use is limited to the measurement of the 
corneal curves on which the existence of astigmatism mainly 
depends, and that it is of no value in determining either 
myopia or hypermetropia, which are due entirely to the 
lengthened or shortened axis of the eye-ball. The detection 
and correction of astigmatism being the most difficult part of 
the refracting optician’s work, and the ophthalmometer being 
the most reliable instrument at his command for this purpose, 
the controversy as to its scope and value is much simplified. 

The principle on which the examination by the ophthal¬ 
mometer is based is the measurement of the curvatures of the 
cornea by means of reflected images viewed through a tele¬ 
scope, the idea being not so much to ascertain the absolute 
curvature of the cornea, as to detect the differences of curva¬ 
ture in its different meridians. 

THE KERATOSCOPE, OR KERATOMETER. 

11. The purpose of this instrument is also to measure the 
cornea, to detect any irregularities 
of its surface, and to deter¬ 
mine the existence of astigmatism. 

It is very simple in its construc¬ 
tion, and depends upon the manner 
in which the cornea reflects these 
concentric rings. If the cornea 
lias the normal spherical curva¬ 
ture, these rings will appear cir¬ 
cular; whereas, if any marked 
degree of astigmatism is present, 
these rings will appear oval, and 
the direction of their longest 
diameter will indicate the defective 
meridian. 



i8o 


OUTFIT REQUIRED. 


THE PHOROMETER. 

12. The condition of the ocular muscles is an important 
feature in the examination of almost every case that applies to 
the optician, and this can be best determined by the use of a 
phorometer, which enables the tests to be made quickly and 
accurately. By means of this instrument the optician is able 
not only to measure the muscular equilibrium and detect any 
departure from the normal condition, but at the same time 



and with equal facility to ascertain the strength and position 
of the correcting prisms. 

This is scarcely the place to urge upon the optician the 
necessity of an examination of the muscular system of the eve, 
as a routine practice in every case of any magnitude that 
applies for correction. And while the condition of the 
muscles and the existence of any insufficiency can be deter¬ 
mined by the Maddox lenses or even by simple prisms, yet the 
advantages of a phorometer are self-evident. 





OUTFIT REQUIRED. 


l8l 


AN ADJUSTABLE OPTICAL BRACKET. 

13. There has always been more or less objection on the 
part of the patient to the use of the trial-frame, which is com¬ 
plained of as being heavy and uncomfortable, the pressure on 
the nose being unbearable if the examination is protracted, 
while the optician finds it troublesome to adjust to the patient’s 
face and difficult to keep in place. To meet these objections 
an adjustable bracket with a test-lens holder, which can be 



firmly attached to the wall, the window sill, or the back of a 
heavy chair has been devised by Dr. Charles A. Oliver. 

In connection with the test-lens holder and its use in 
measuring the refraction of the eye with trial-lenses, it also 
contains a phorometer; and the advantages of having this 
instrument in a convenient form for quickly placing in front 




















OUTFIT REQUIRED. 


182 

of the eye, and in a more rigid position than is possible with a 
stand or tripod, are obvious. 

PERIMETER. 

14. A perimeter has been included in the optician’s outfit, 
but it is not urged as an actual necessity. It is an instrument 
rather for the use of the physician than the optician, although 



the latter can also profit by the information gained by its use. 
It has been devised for testing the field of vision, to ascertain 
its extent and to detect the existence of any blind spots. It, 
therefore, has nothing to do with direct vision, or with the 
vision of the yellow spot, or with the vision that can be cor¬ 
rected with glasses; but deals only with indirect vision, or the 
vision of the retina outside of the yellow spot. Detailed direc¬ 
tions for its use will be given at another place in this work. 








CHAPTER IX. 


METHOD OF EXAMINATION. 

We have now passed through the theoretical portion of 
our subject, and this brings us to the practical part. The 
foregoing chapters have been devoted to theory ( so-called), 
while the remaining chapters will treat of practice; or, in other 
words, the balance of the work will show how to put into 
practical use what was learned in the former chapters. We 
have treated, with considerable attention to detail, the theo¬ 
retical subjects embraced in this treatise. To many opticians 
these matters may have seemed dry and uninteresting, and 
perhaps some may have even thought them unimportant; but 
they are the necessary foundation, deeply and securely laid, 
without which the lasting superstructure of a successful opti¬ 
cian cannot be builded. That is to say (and the statement 
cannot be made too strong), no optician can make a reputation 
for himself and build up a growing optical business, unless 
he is grounded in the scientific principles of his profession. 
Otherwise, where is he better than the country storekeeper 
who sells glasses, or the itinerant peddler? Otherwise, why 
should he claim the title of “optician” ? An optician is really 
a person skilled in the science of optics, and this is the original 
meaning of the word; but in later years the significance of 
the word has been changed, and the title assumed by any 
dealer who keeps for sale a stock of spectacles and eye-glasses, 
however small. This should not be, but only those persons 
should be called opticians who have had special training and 
acquired special knowledge and skill, and no reader of this 
work should content himself until he has measured up to this 
standard. 


183 



METHOD OF EXAMINATION. 


184 


The science of the examination of the eye in a healthy and 
diseased condition is of recent growth. It might be called the 
“metrology” of the eye, which literally means a discourse on 
the measurement of the eye. It is only within the past fifty 
years that the physician was able to ascertain much more than 
the simple fact of perfect vision, or impaired vision, or no 
vision at all. In the latter class, two grand divisions were 
included under the names of amaurosis and amblyopia, which 
Landolt says were differentiated as follows: “In amblyopia 
the patient saw nothing, but the physician saw something; in 
amaurosis, neither the physician nor patient saw anything.” 
That is to say, the word amblyopia was used to include all 
those visual defects which were produced by changes visible to 
the eye of the physician, such as opacities of the cornea, etc.; 
while, on the other hand, all those diseases which had their 
origin and seat in the interior of the eye, such as retinitis, 
choroiditis, atrophy of the optic nerve, etc., and which were 
not appreciable to the unaided eye of the physician, were in¬ 
cluded under the general head of amaurosis. 

The first important work in the line of the examination of 
the organ of vision was accomplished by an Englishman by 
the name of Thomas Young, who published the results of his 
studies in the “Philosophical Transactions,” 1793, one hundred 
years ago. He seems to have been a man of unusual learning 
for his day, and his views were so much in advance of his 
time that they were neither understood nor believed by the 
scientific men of that period. It is an interesting fact that 
this same man, Thomas Young, was affected with a considera¬ 
ble degree of astigmatism, which he himself studied and cor¬ 
rected—this being the first case of the kind that had ever been 
analyzed or corrected. 

After this, many scientific men labored in the same field 
and worked along the same lines, one by one adding little by 
little to the stores of scientific ophthalmology, increasing and 
perfecting the means of examining the eye, evolving the laws 
governing the functions of refraction and accommodation, and 
making practical application of them in the detection and cor¬ 
rection of the various optical defects with which they had to 
deal. This brings us down to Snellen, who devised his classi- 


METHOD OF EXAMINATION. 


185 


cal test-types, which really formed the first scientific method 
for the determination of the acuteness of vision. Then fol¬ 
lowed the introduction of the ophthalmoscope, the perimeter, 
and the examination of color perception, the addition of which 
makes our means of examination of the visual apparatus very 
satisfactory and quite complete. 

In the examination of the eyes, as in any other scientific 
undertaking, we should follow some definite plan and conduct 
our examinations in a systematic manner. This is absolutely 
necessary in order that nothing may be overlooked, and that 
we may accomplish our purpose with a reasonable certainty. 

Physicians divide their examination of any portion of the 
body into objective and subjective , and we shall observe the 
same division in our examination of the eye. The object of the 
objective method of examination is to reveal the condition of 
the eye in a state of rest, while the subjective method is de¬ 
signed to make us acquainted with the eye in a state of func¬ 
tional activity—that is, to determine the condition of the func¬ 
tion of vision. These two terms are used one opposed to the 
other, and it is important that the reader have a clear under¬ 
standing of them. 

According to the dictionary, the term subjective is applied 
to those internal states of thought or feeling of which the mind 
is the subject; opposed to objective , which is applied to things 
considered as separate from the mind and as objects of its atten¬ 
tion. Thus subjective truth is that which is verified by con¬ 
sciousness; objective truth, that which results from the nature 
and relations of things. A subjective motive is an internal 
feeling or propensity; an objective motive is something external 
to the mind. 

Applying these conditions to the examination of the eyes, 
a subjective examination is one which can only be made with 
the assistance of the patient, and depends on the information 
elicited by the patient’s answers. An objective examination 
can be made without the patient’s cooperation, without asking 
him a single question, and even against his own wishes, as it 
depends on the information which the observer is able to gain 
from an external examination of the conditions present, and 
which are visible or appreciable to his educated senses. A 


186 


METHOD OF EXAMINATION. 


subjective examination reveals information which must come 
from the mind of the subject, while an objective examination 
reveals such conditions as are objects of the observer’s atten¬ 
tion. No examination is complete unless it is both subjective 
and objective. 

The objective examination usually comes first, and begins- 
with a general inspection, which is accomplished at a glance, 
of the appearance of the head, and face, and eyes of the patient. 
Now the shape of the head and face and the form of the eye 
sometimes indicate the condition of refraction present. Hv- 
permetropia is due to a want of development or an arrested 
development of the eye in length; while myopia, on the con¬ 
trary, depends on an over-development or an elongation of the 
eye-ball in its antero-posterior diameter. This difference in 
the form of the eye-ball is sometimes accompanied by changes 
in the shape of the head and face, and indicated by the appear¬ 
ance of the features of the patient; a flat face indicating hyper- 
metropia, and a full face and prominent eye-balls indicating 
myopia. 

A lack of symmetry in the two sides of the face, one side 
being more prominent than the other, would point to the pos¬ 
sible existence of anisometropia, a condition in which there is 
a difference in the refraction, one eye being myopic and the 
other hypermetropic. Other forms of asymmetry of the face,, 
such as a deviation in the median line and a lateral curvature 
of the nose, often occur in persons who are astigmatic as well 
as anisometropic. 

The appearance of the two eyes should then be compared 
for the purpose of noting whether there is any difference be¬ 
tween them, and for the purpose of detecting any slight diver¬ 
gence or convergence of one eye, or any inequality in the size 
or dilatation of the two pupils, or any protrusion of one eye 
more than the other, or any drooping of either eyelid. A 
difference in the size of the opening of the lids can be readily 
recognized in this way. 

A careful glance at the edges of the lids comes next, to 
determine the existence of any swelling or inflammation, and 
also to note the position of the lashes, which is a very import¬ 
ant matter as regards the welfare of the eye and the trans- 


METHOD OF EXAMINATION. 


i8 7 


parency of the cornea. Displaced eyelashes, or lashes with 
their point directed toward the eye-ball and scraping' over the 
sensitive surface of the cornea, will set up and keep up a condi¬ 
tion of irritation and inflammation that will neutralize the 
benefits to be derived from the most carefully-adjusted glasses. 
And again, it should be remembered that an inflammation and 
swelling of the lids may be due to the strain caused by an 
uncorrected optical defect, and be speedily cured by the proper 
glasses. 

The distance between the two eyes is a matter that should 
not escape the attention of the careful observer, because of the 
disturbing effect it may have on the act of convergence. It is- 
a self-evident fact that in converging the eyes for vision at any 
given distance, the more widely the eyes are separated the 
greater must be the effort at convergence; while the closer the 
eyes are placed together the less convergence will be called for. 
In this way, in a certain number of persons fixing the eyes for 
vision at a given distance, there will be a great variation in 
the angle of convergence that is required for each person. As 
convergence is an act that is being constantly brought into' 
play, it can easily be seen that any increase in the effort re¬ 
quired (however slight) may be the source of great discomfort, 
and the overlooked cause of many cases of asthenopia. Prof. 
Landolt, the eminent French oculist, says: “For my part I am 
thoroughly convinced that the insufficiency of the internal 
recti muscles, which is such a frequent cause of the asthenopia 
of myopic patients, is due in many cases to an excess of the 
distance between the two eyes.” 

This is a subject Which is almost entirely overlooked, 
and yet it is one that deserves the most careful attention. It 
seems like a very simple matter to measure the distance be¬ 
tween the eyes, and yet when we inquire into the question 
carefully, we will find that to make an exact measurement is 
attended with no little difficulty. If we measure the distance 
between the centers of the pupils, as is commonly done, there 
may be two sources of error. In the first place, the pupil is 
not exactly in the center of the anterior part of the eye-ball, 
but is a trifle to the inner side. This is an anatomical point 
that is not always borne in mind, as we are apt to take it for 


188 


METHOD OF EXAMINATION. 


.granted, as a matter of course, that the center of the pupil 
represents the center of the eye-ball, and many opticians have 
never known anything different. The second element of error 
lies in the difficulty of measuring the exact distance between 
the pupils, as this is a point that can only be determined 
approximately. 

Another chance for error lies in the fact that the distance 
between the two eyes or between the pupils should represent 
the distance between the eyes when they are directed in lines 
parallel to each other; and here is where the difficulty lies, for 
it is almost a matter of impossibility for any patient to direct 
his eyes absolutely parallel; there always remains a greater or 




less degree of convergence, the amount of which cannot be 
determined. This objection is particularly applicable in cases 
of strabismus, where it is evident the eyes can never be 
brought to a state of parallelism, and yet these are the very 
cases where it is most desirable to measure the intraocular 
distance. 

We have been considering the distance between the eyes 
with regard to its effect upon convergence and the relation 
it may bear in the causation of many cases of asthenopia, a 
matter too often overlooked. Of course, all opticians are 
accustomed to make a simple approximative measurement 
when they want to determine the interpupillary distance, or 
the distance which should separate the glasses of spectacles, 
for which purpose an ordinary graduated rule may be satis¬ 
factorily employed. The patient is directed to look straight 







METHOD OF EXAMINATION. 189* 

before him at some distant object, so as to make the axes of 
vision as nearly parallel as possible. 

The measuring rule or yard-stick is placed across the 
nose on a line with the center of the pupils, as close to the 
patient s eyes as possible, and in such a position that the end 
will be directly in front of the center of one pupil. The point 
of the rule that is directly in front of the center of the pupil of 
the other eye may be marked by the optician’s thumb-nail, and 
the distance between these two points may be read off the rule, 
and the result will give the pupillary distance. 

The optician should stand as far away from the patient's- 
face when he makes this measurement as the length of his arm 
will permit, so as to diminish as much as possible the error 
that may be caused by the convergence of his own eyes. The 
optician’s eyes will scarcely be more than two feet away from 
his patient’s eyes, and the lines converging from his own eyes 
towards the patient’s eyes will make the interpupillary dis¬ 
tance a little less than the real distance. It has been calcu¬ 
lated that the error in measuring the pupillary distance by this- 
method is about one line (one-twelfth of an inch), which 
should be added to the apparent distance in order to obtain the 
correct measurement. While this method, if carefully em¬ 
ployed, answers every purpose and its results are sufficiently 
satisfactory, yet a still greater degree of accuracy may be in¬ 
sured by adding about the sixteenth of an inch for distance and 
deducting the same amount for reading. For instance, if the 
distance on the scale was two and three-sixteenth inches, we- 
would add one-sixteenth for distance, which would make two 
and four-sixteenth inches or two and one-quarter inches; for- 
reading we would deduct one-sixteenth inch, which would 
leave two and two-sixteenth or two and one-eighth inches.. 
The optician will understand that this is done to make the 
optical centers of the lenses correspond to the pupillary dis¬ 
tance, which varies with the increased and diminished con¬ 
vergence required in reading and distance. 

Another method of securing accuracy and of obviating 
the error that may be caused by the convergence of both 
patient’s and optician’s eyes, is to measure with each eye sepa¬ 
rately as follows: The optician is seated directly in front of the 


190 


METHOD OF EXAMINATION. 


patient, who is asked to fix his eyes on some distant object 
which will give them an essentially parallel direction. The 
rule is then applied across the patient’s nose as before, with its 
end directly in front of the center of the pupil of the right eye, 
the exact position of which is determined by the optician’s left 
-eye alone, his right eye being closed. Then the optician’s left 
eye is closed and the right eye is opened in order to read off 
the point on the rule which is exactly opposite the center of 
the pupil of the patient’s left eye. This method requires that 
during the examination the optician should keep his head and 
•eyes absolutely motionless. 

Pupilometers are special instruments designed for measur¬ 
ing the interpupillary distance. There are different forms on 
the market, but they are all made on the same principle, and 
consist essentially of projecting points, one or both of which 
slide on a scale, and can be placed at any desired distance 
apart, one in front of the center of each pupil, while the dis¬ 
tance between them is read off the scale on the instrument. 
To rectify the result obtained by one of these instruments, the 
same correction must be made as when a simple graduated 
rule is used. 

The next point to be noted in the examination of a pa¬ 
tient’s eye is the amount of protrusion of the eye-ball, which is 
sometimes a symptom of very great importance, and the prin¬ 
cipal symptom in a number of diseases of the eye or orbit. 
When there is any inflammation or swelling, or tumor or 
growth of any kind pressing upon the ball, it protrudes out¬ 
ward, because, being encased in the bony orbit, that is the only 
direction in which it can recede before the offending material. 

If the protrusion attains such a degree that the lids no 
longer suffice to entirely cover the surface of the cornea, the 
condition is a serious one on account of the pernicious effects 
it entails upon this membrane and other portions of the eye. 
The cornea becomes dry, rough and scaly, then ulcerates and 
becomes opaque, until it is finally destroyed and blindness 
results. 

This condition of protrusion is known as exophthalmus, 
and can only be measured approximately and without any 
great precision. It usually suffices to gain a relative knowl- 


METHOD OF EXAMINATION. 


191 

-edge of its degree as compared with the normal condition; or, 
if the exophthalmus is great enough to prevent closure of the 
lids, then we notice the size of the palpebral opening. 

The eyes of myopic patients are large and prominent, with 
• occasionally some protrusion, but scarcely enough so to justify 
the term exophthalmus. 



Illumination of Cornea. 

[From Wells’ Diseases of the Eye, by Bull.] 


The condition of the conjunctiva next attracts our atten¬ 
tion, and we notice whether it is clear and transparent, or con¬ 
gested and inflamed. Attention should next be given to the 
condition of the cornea. The chief characteristics of this 
membrane are its uniform transparency and its smooth reflect¬ 
ing surface, which, as a convex mirror, affords an erect but 
diminutive image of the objects placed in front of it. In the 
healthy eye the transparency of the cornea allows us to see the 
fibrillations and the color of the iris with perfect distinctness. 
These may be concealed, or seen but imperfectly, either on 
account of some opacity of the cornea, or on account of tur¬ 
bidity or cloudiness of the aqueous humor, due to the presence 
of blood or inflammatory products in the anterior chamber. 
When the cornea itself is cloudy, the cloudiness or opacity is 
at first seldom uniform over the whole surface, and hence the 
more transparent portions will often allow us to see the con¬ 
dition of the aqueous humor and of the iris beneath. Slight 
opacities of the cornea are liable to escape a cursory examina¬ 
tion by reason of the direction in which the light falls upon 
them, or because they resemble in color the background of the 
iris or pupil behind them. In order to get the best possible 
view of the cornea, the patient should be seated facing a good, 
large window admitting plenty of diffuse daylight, but not ex- 





192 


METHOD OF EXAMINATION. 


posed to the direct rays of the sun. The eye under inspection 
should not only be turned successively in various directions, 
but the optician should also vary his own point of view; and 
in any case of doubt, he should employ a convex lens to con¬ 
centrate light upon the parts observed, and, if necessary, a 
second convex lens to act as a magnifier, with which the better 
to examine the illuminated surface. Or the same procedures 
can perhaps be more satisfactorily done by the use of lamp¬ 
light. 



Use of Illuminating and Magnifying Lenses. 
[From Wells’ Diseases of the Eye, by Bull.] 


Inspection of the cornea in this manner may show that its 
surface is more or less cloudy, or irregular in shape, or ulcer¬ 
ated, or marked by the cicatrices of former ulcers, all of which 
conditions will materially affect the sight, and in such a way 
as not to be remediable by glasses. Opacities of the cornea 
will appear by this oblique illumination (which is reflected 
light) of a grayish or whitish color, while the same spots seen 
with the ophthalmoscope (which is transmitted light) will 
appear as dark spots upon a bright red background. It has 
many times happened that an optician has spent considerable 
time in fruitless efforts to find a lens that will improve his 
patient’s sight, much to his own discomfiture and his patient’s 
disappointment, when finally a chance glance at the cornea. 






METHOD OF EXAMINATION. 


193 


shows a cloudiness or opacity that cannot be remedied by any 
combination of lenses. 

We next examine the aqueous humor, which should be 
perfectly clear and transparent like water. When the cornea 
and aqueous humor are both transparent, they allow the con¬ 
dition of the iris and of its central aperture, the pupil, to be 
clearly seen. 

The chief characteristic of the healthy iris is its lustrous 
striated surface; while the chief characteristics of the healthy 
pupil are its circular outline, its free mobility, and its clear, 
bright blackness. After fifty years of age this blackness is 
usually exchanged for a more or less grayish or yellowish tint, 
due to the senile changes in the crystalline lens. 

Inasmuch as the pupil contracts and dilates with the in¬ 
crease and diminution of tension of the accommodation, it can 
be seen that the mobility and diameter of the pupil may afford 
indications as to the state of the accommodation. 

This inspection or examination of the exterior of the eye, 
which has occupied so much space and taken so long to de¬ 
scribe, can, after a little practice, be completed in a few 
minutes, and we are then ready to proceed with the examina¬ 
tion of the refraction and visual acuteness. 

The optician should ask his customer how and in what 
way his eyes trouble him. Don’t put the question in such a 
way as to ask what is the matter with his eyes, or he may reply 
that that is what he wants you to tell him; but the question 
should be framed in such a shape that he will be led to tell the 
optician the train of symptoms of which he has to complain. 

Unless the patient is too prolific, and is disposed to take 
up too much of your time with his tale, it is well to patiently 
listen to him, and allow him to make his statement in his own 
language. He may say that he sees well enough at a distance, 
and that his principal difficulty is in reading, especially in the 
evening; or that after reading awhile the print runs together, 
or the eyes water; or perhaps he is compelled to stop awhile 
and close his eyes and rub them before taking a fresh start. 
The optician will at once recognize these symptoms as point¬ 
ing toward hypermetropia as the probable cause of the trouble, 
or, in a middle-aged person, toward presbyopia. 


194 


METHOD OF EXAMINATION. 


Or fie may say that he has no difficulty in reading or writ¬ 
ing, or any close work, but that he cannot recognize his 
friends across the street, or that he cannot see the expression 
of the preacher’s face in church, which symptoms would at 
once lead the optician to suspect myopia. Or perhaps the 
complaint may be that vision is not satisfactory for either read¬ 
ing or distance, with pain and discomfort on using the eyes, in 
which case astigmatism would be suspected. 

In any case of imperfect vision, it is desirable to deter¬ 
mine in the commencement of the examination whether the 
defective sight is due to> some error of refraction, or to some 
organic disease of the eye. Fortunately we possess in the 
“pin-hole test” a method by which this point can be easily and 
quickly determined. 

A black metal disk having a small pin-hole perforation in 
its center (such as is found in all the more complete trial-sets 
of test-lenses) is given to the person, and he is told to hold it 
quite close to the eye under examination, care being taken to 
see that the pin-hole is directly in front of the center of the 
pupil. This admits into the eye a small pencil of the rays of 
light, which passes through the axis of the refractive system of 
the eye, forming a clearly-defined image for all distances. If 
this noticeably improves vision on the distant test-card, then it 
is known that the refractive system of the eye is at fault, and 
that a similar or greater improvement in sight can be expected 
from glasses; but if, on the contrary, vision is not at all im¬ 
proved by the pin-hole disk, then some organic disease of the 
eye may be suspected, which is not remediable by glasses, and 
which removes the case beyond the province and aid of the 
optician. The pin-hole disk can be used in the trial-frame 
over one eye, while the other is excluded by the opaque disk; 
but the suggestion was made above to allow the patient to hold 
it in his hand, and in this way it perhaps could the better be 
placed directly in front of the pupil. 

The pin-hole test is a simple method, as reliable as it is 
simple, and we always advise opticians who read these lines to 
use the pin-hole test in every case of defective vision. If the 
optician thus discovers early in his examination whether or 
not it is a case of refractive error he has to deal with, he will 


METHOD OF EXAMINATION. 


T 95 


save much valuable time (which he might otherwise spend in 
trying to fit a case that could not be helped by glasses), and he 
will be in a position to give reliable advice if it is a case which 
does not come within his province to be fitted with glasses. If 
the optician finds it to be a case of organic disease, he fails to do 
his duty unless he advises the patient to consult a physician; 



Pin-hole Disk. 


while if the pin-hole test shows it to be a refraction case, the 
optician will continue his examination by following the 
methods given below. 


EMMETROPIA 

As it is proposed in this work to treat of vision and the 
errors to which it is subject, and as in former chapters the 
various anatomical parts concerned in the sense of sight have 
been explained and demonstrated,, and also the effect of lenses 
on rays of light, we are now in a position to consider the eye as 




196 


METHOD OF EXAMINATION. 


an optical instrument, and to examine the normal eye in its 
power to see and appreciate the objects all around us. 

The normal eye will be mentioned first, although we be¬ 
lieve that there are very few persons who possess perfectly 
normal eyes and vision even from their birth, although doubt¬ 
less there are many such persons who have had no trouble with 
their eyes, and who have always supposed their sight to be 
equal to that of the perfect standard. 

By the normal or emmetropic eye is meant one that, when 
in a state of rest, has its refractive power so adjusted that it 
can see distinctly at a remote point or infinity; so that parallel 
rays of light are brought to an exact focus on the retina at the 
yellow spot without any effort on the part of the eye or its 
accommodation. This is called emmetropia, meaning that the 
eye is in measure. 

If an emmetropic eye be tested, what will be the result of 
the examination as regards the vision at a distance and at the 
near point? The vision will be f§- or one, and the nearest 
point of distinct vision for the smallest type will be about five 
inches, if the person examined be not over twenty years of age. 
The cornea, aqueous and vitreous humors, with the crystalline 
lens, are the refractive media of the eye. Combined, they 
represent a bi-convex lens of rather less than one-inch focus. 
The optical center of the combination is a little behind the 
crystalline lens, and the retina is a sensitive screen placed at 
the focus of the combination. 

Our perception of the outer world is due to the formation 
of real inverted images on the retina, although we are uncon¬ 
scious of the inversion. In order that objects may be seen 
distinctly, their images must be accurately focused on the 
retina. We have already considered the change that takes 
place in the eye by which we are enabled to tell the time on 
the distant tower-clock, and compare the figures there with 
those on our own watch held in our hand, which power of 
adjustment for near and far vision is known as the accommo¬ 
dation of the eye. 

The principles involved in the construction of an emme¬ 
tropic eye may be appropriately illustrated by a reference to the 
magic lantern or stereopticon, with the use of which every one 


METHOD OF EXAMINATION. 


197 


becomes more or less familiar in his boyhood days. We soon 
learned in using this instrument that the screen must be at the 
proper distance; that the lens must be of the proper refractive 
power, and must be moved in or out to the proper position, in 
order to produce a clear and distinct image upon the screen. 
In other words, the power and position of the lens and the 
distance of the screen bear certain fixed relations to each 
other, which relations cannot be altered without marring the 
clearness of the image produced. 

The same relation exists between the refracting media of 
the eye (corresponding to the lens of the magic lantern) and 
the retina (corresponding to the screen on which the magic 
lantern pictures are thrown). A clear image can be formed on 
the retina only when the refracting media are of the proper 
refractive power, and the retina situated at their proper focal 
distance, in which case the parallel rays of light entering the 
eye are brought to a focus exactly at the position of the retina, 
that is, on the retina. In the construction of such an eye, all 
the conditions having been complied with in conformity with 
the laws of optics, a perfect image is necessarily formed on the 
retina, and this being conveyed to the brain by the optic nerve 
produces normal and comfortable vision. 

Any disturbance between these relations blurs the dis¬ 
tinctness of the image formed and produces abnormal vision, 
and this disturbance may result as follows: either the refract¬ 
ing media of the eye may have too much or too little refractive 
power, or the retina may be too near or too far from the lens, 
or sometimes both conditions may be found in the same eye. 

Thus the significance of the word emmetropia can be ap¬ 
preciated, which means “an eye in measure.” On the other 
hand, when the relation between the distance of the retina and 
refracting power of the media does not correspond, the condi¬ 
tion is very appropriately called ametropia, which means “an eye 
out of measure.” The student should clearly understand the 
significance of these two words, so that when in his reading he 
encounters either one of them, there will at once flash into his 
mind a perfect understanding of the conditions involved. 

Thus it will be seen that the length of the eye-ball, or, in 
other words, its axial diameter, plays a very important part in, 


198 


METHOD OF EXAMINATION. 


and, in fact, is the fundamental condition of, two of the 
common errors of refraction which the optician is daily called 
upon to correct with lenses. 

The conditions present in emmetropia and ametropia, and 
the meaning of in measure and out of measure, can be perfectly 
demonstrated by an experiment: 


HEM 



Focus of parallel rays of light in Emmetropia as compared 
with Hypermetropia and Myopia. 


A strong convex lens is taken and held in such a way that 
rays from distant objects will pass through it and be refracted, 
and form an image on a piece of cardboard held at the princi¬ 
pal focus of the lens. The card may be moved nearer to and 
farther from the lens until the exact position is found where 
the most perfect image is formed. If a convex lens of ten 
dioptres is used, this position will be at four inches from the 
lens, and everything is exactly “in measure,” and this corre¬ 
sponds to the condition known as emmetropia. 

The card may now be moved nearer to the lens, and then, 
instead of a perfect image being formed, there will be seen 
circles of diffusion, because the rays strike the card before 
they have had an opportunity to converge to a focus, and 
hence everything is “out of measure.” This corresponds to 
the condition known as hypermetropia. If another convex 
lens be interposed it will add to the strength of the original 
lens, and thus bring the rays to an earlier focus; and if the 
additional lens be of the proper strength, this focus will be just 
at the point where the screen is placed. 

If, on the other hand, the card be moved farther away, a 
perfect image can no longer be formed, and again we see noth¬ 
ing but diffusion circles or a confused patch formed on the 
card, because it is farther away than the principal focus of the 












METHOD OF EXAMINATION. 


199 


lens, and the rays only strike it after they have converged to a 
focus and then diverged. Here, again, everything is “out of 
measure,” and this corresponds to the condition known as 
myopia. Here the rays have come to a focus too soon; and if 
a concave lens be now interposed, it will detract from the 
strength of the original lens, and thus bring the rays to a later 
focus; and if this concave lens be of the proper strength, this 
focus will be just at the point where the card is placed. 

These illustrations explain how hypermetropia can be 
corrected by convex lenses, and myopia by concave lenses, 
and that emmetropia needs no correcting lens. It should be 
further stated that the above remarks about the emmetropic 
eye and the ametropic eye all refer to it when it is in a state of 
rest; that is, when there is no accommodative effort, and when 
the eye is adjusted for parallel rays or for distance. Or, in 
other words, that eye only can be considered as emmetropic 
that exactly focuses the rays of light upon its retina when in a 
state of rest; and that eye ametropic that is unable to thus 
focus the rays when passive or at rest. 

An emmetropic eye, then, is said, in a state of rest, to be 
adjusted for infinity, because it is adjusted for parallel rays, 
and only those rays are absolutely parallel that proceed from 
infinity. But in reality, and for all practical purposes, any 
object situated at fifteen or twenty feet or beyond emits rays 
that are so nearly parallel, and that cannot be demonstrated to 
be not parallel, that they are called parallel, and as such are 
used for determining the refraction of an eye, that is, whether 
it is emmetropic or ametropic. And while the expressions 
infinity and infinite distance are frequently used, yet in practice 
we commonly use a distance of twenty feet without disturbing 
the accuracy of our calculations. 

The “refraction of the eye” and the “acuteness of vision” 
are frequently confounded. They are two very different 
things, however, and the optician should have a clear under¬ 
standing of the difference between them, and how to be able to 
clearly distinguish one from the other, although in practice :* 
is customary to determine them together. In simple words, 
the refraction is the function of the dioptric apparatus of the 
eye; while, on the other hand, the acuteness of vision is th3 


200 


METHOD OF EXAMINATION. 


function of the nervous system of the eye. The refraction of 
the eye refers to the action of that organ on the rays of light 
that enter it, while acuteness of vision has reference to the 
image formed on the retina, and to the sharpness of sight as 
perceived by the brain. 

Refraction may be perfectly normal, and still the eye not 
be able to see, if the nervous apparatus fails to perform its 
function properly; that is, the dioptric apparatus may be such 
as to exactly focus the rays on the retina and form an image 
there; but no matter how perfect the image may be that is 
formed there, if it is not conveyed to the brain by the optic 
nerve there can be no resulting vision. While, again, the 
acuteness of vision may be normal, in spite of great anomalies 
in the refraction, if these latter are properly corrected by 
glasses. While there may be refraction without acuteness of 
vision, there can be no acuteness of vision without refraction. 
Refraction is the first step, or cause—acuteness of vision is the 
second, or the result. 

The refraction of all eyes, even of the eyes of a dead 
person or the enucleated eye of any animal, can be deter¬ 
mined; but, on the other hand, the acuteness of vision can be 
determined only for a living eye, for the determination of 
which, however, it is necessary that its possessor be able to ex¬ 
press himself clearly in regard to the luminous impressions 
received on his retina and conveyed to his brain. Refraction 
may be an objective symptom, while acuteness of vision is a 
subjective symptom. 

Acuteness of vision is for the retina what tactile sensibil¬ 
ity is for the skin, and the condition of the two functions is 
determined in an analogous manner. We seek in both for the 
smallest distance between two points which can be perceived 
separately. For the skin, the mechanical pressure of the two 
points of a pair of compasses is made use of; for the retina, 
the impressions produced by the retinal images of two lumin¬ 
ous points. The determination of the acuteness of vision con¬ 
sists, therefore, in the determination of the smallest retinal 
image, the form of which can be distinguished. 

We desire here to call the attention of the optical student 
to a distinction and a difference—it is not simply the smallest 


METHOD OF EXAMINATION. 


201 


retinal image that can be perceived that gives the measure of 
the acuteness of vision, but it is the smallest retinal image 
whose form can be distinguished. The perceptibility of the 
smallest retinal image depends solely on the luminous intens¬ 
ity of the point that produces the image. One luminous point 
does not measure the visual acuteness or the distinction of 
forms, but simply the perception of light, or the faculty which 
the retina possesses of distinguishing differences of brightness. 
The acuteness of vision, or the faculty of the retina to perceive 
forms, depends on several conditions: 

1. Primarily on the sensibility of the retina. 

2. On the adaptation of the retina. 

3. On the general illumination. 

4. On the sharpness of the retinal image. 

5. On the intensity of illumination. 

The adaptation of the eye to the illumination under which 
it acts is a condition which it becomes necessary to take into 
account in all experiments relative to the distinction of the 
degrees of clearness and of colors. 

In passing from an illumination of a less to one of a 
greater intensity, or inversely, it takes a certain length of time 
for the retina to become accustomed to the altered illumina¬ 
tion and to put itself in harmony with it. We know that the 
acuteness of vision varies with the general illumination as that 
of a clear, sunny day compared with that of a dark, cloudy 
day. This is patent to every oculist and optician who uses 
diffused daylight in the examination of his cases, and causes 
variation and confusion in the records of those cases which 
are examined on several different days, when these varying 
conditions of brightness and gloominess prevail. 

The sharpness of the retinal image depends essentially on 
the transparency of the dioptric media, the regularity of their 
surfaces, and the adjustment of the eye to the distance of the 
object. The results of numerous experiments have proven 
that it is essential that the two points of a retinal image, in 
order to be clearly distinguished one from the other, must be 


202 


METHOD OF EXAMINATION. 


separated by a certain small distance. Such a retinal image, 
that is, the space or distance by which these two points are 
separated, corresponds in the normal emmetropic eye to a 
visual angle of one minute. 



Test-types of Snellen. 


The test-types of Snellen are those in most common use 
for the determination of the acuteness of vision, and he has 
based his types on the point mentioned above (that a visual 
angle of one minute is the smallest that can be perceived), and 
he has made a selection of such letters that the width of each 
line of a letter, at its proper distance from the eye, will form 
two opposite points on the retina, which, with the nodal point, 
subtends an angle of one minute; that is, each of the black 
lines of every letter will form an image of that size in its 
smallest part. Now if we take five lines or five spaces for the 
size of each letter, we will then have formed an image on the 
retina which will subtend an angle of exactly five minutes. 
For the majority of cases this is the smallest image and forms 
the smallest visual angle that the normal retina can comfort¬ 
ably appreciate. Each series or size of Snellen’s letters is 
marked by a number which indicates in feet the distance at 
which the letters appear under an angle of five minutes, and 
the lines of the letters at an angle of one minute. 

It may, perhaps, at this point be well to stop long enough 
to explain the meaning of the terms degrees, minutes and 
seconds. A degree is expressed by this sign (°), a minute by 
this sign ('), and a second by this ("). These are matters on 
which a great many students have very confused ideas. Every 
circle, no matter how large or how small it may be, is divided 
into 360°, each degree is divided into 60', and each minute into 
60", a second being the smallest subdivision. If the circle is 








METHOD OF EXAMINATION. 20^ 

large these subdivisions can be very easily seen, but if the 
circle is small the subdivisions become microscopical. In a 
circle of one foot in diameter, we can easily see the divisions- 
into degrees, and can possibly detect the subdivisions into 
minutes, but it will require a magnifying glass or a microscope 
to detect the dimensions of an angle of one second. On the 
other hand, it has been stated that if a circle is taken with a 
radius of the moon’s distance from the earth, it can be sub¬ 
divided into seconds which will each represent a distance or 
space of one mile. A little reflection will show that a degree r 
or a minute, or a second does not represent any fixed or exact 
space or size in inches or feet, but must be considered in con¬ 
nection with the diameter or radius of the circle, be it large or 
small, and as only indicating a fixed and definite proportional 
part of that circle. This is an important point to remember,, 
as otherwise great uncertainty and confusion might arise in 
the mind of the student when these divisions of a circle are 
mentioned. 

As an illustration of the comparative size of degrees, min¬ 
utes and seconds, we might take the letter that should be visi¬ 
ble at a distance of one hundred feet. The diameter of this 
circle would be two hundred feet and the radius one hundred 1 
feet, and the circumference a little more than six hundred feet. 
This six hundred feet is the length of an imaginary circle one 
hundred feet away, with the axis of vision as the center of the 
circle. If we divide this by three hundred and sixty we will 
find the height of one degree; and if we divide this by sixty we 
will find the height of one minute, which is just the width of 
one of the lines or spaces of the letter. 600 divided by 360 = 
if feet as the size of one degree, if divided by 60 = \ foot 
as the size of one minute. 

The same illustration may be applied to the reading type, 
which we will suppose is located at fifteen inches, which is the 
proper distance to hold a book for continuous reading. This 
gives us the radius of the circle, the diameter of which would 
be thirty inches, and its circumference (being a little more 
than three times its diameter) about ninety inches. This cir¬ 
cumference of ninety inches is divided into three hundred and 
sixty degrees (as every circle is), which gives us one-fourth 


204 


METHOD OF EXAMINATION. 


inch as the size of one degree. This quarter inch is divided 
into sixty minutes, which gives us inch as the size of one 
minute. That is, in a circle of thirty inches diameter and 
ninety inches circumference, an angle of one minute is 
inch wide at its base. This, then, is the width or size of the 
smallest object that can be discerned at a distance of fifteen 
inches; while five times this size, T inch or ^ inch, is the 
size of the smallest letter that can be pleasantly and comforta¬ 
bly seen at this distance. Therefore, the letters are so con¬ 
structed that the width of each line or limb of a letter shall 
subtend an angle of one minute, while the letter itself shall 
subtend an angle of five minutes. 

This is the principle that governs the size of the letters, 
and it was determined by no other way than by repeated ex¬ 
periments. Long-continued observations seemed to prove 
that the normal eye was not capable of discerning any object 
that did not subtend an angle of one minute, while for com¬ 
fortable and continuous vision it was necessary that the object 
be five times as large. This is the principle that Snellen put 
to practical use in the construction of his test-letters. 

The illustration below shows that the visual angle is 
always the same for the various letters at their respective dis¬ 
tances. The angle produced by the large letters at one hun¬ 
dred feet is the same as that produced by the small letters at 
twenty feet, in each case being an angle of five minutes. This 
is the case with all the intermediate letters, as at fifteen feet, 
thirty feet, seventy feet, and at two hundred feet, the angle 
remaining the same in every case. Therefore, we find that an 



eye that can see the twenty feet letters at twenty feet can, with 
equal ease, see the seventy feet letters at seventy feet, and the 
largest letters at two hundred feet. 






METHOD OF EXAMINATION. 205 

The size of the visual angle depends upon two factors— 
the distance of the object and its size. The farther an object 
is removed from the eye, the larger it must be in order that the 
visual angle may remain the same. As this visual angle 
remains unchanged and the different letters form an image 
of the same size, we might think the letters were all of the 
same magnitude, if our experience and judgment did not cor¬ 
rect such an impression and convince us that the letters are of 
various sizes and placed at different distances. 

The smallest distance at which two points can be sepa¬ 
rately distinguished is the measure of the visual acuteness. 
The eyes perceive two stars when ithey are far enough sepa¬ 
rated, while if they are brought closer together the two stars 
blend into one. If the stars are separated by an angle of one 
minute or more, they are perceived as two distinct stars; while 
if they are separated by an angular distance of less than one 
minute they run together, as do the stars composing the 
“milky way.” 

The acuteness of vision (the abbreviation of which is 
usually written V.) is expressed by a fraction, the numerator 
of which denotes the distance at which the card is placed, and 
the denominator the number of the line which can be read. 
For instance, if the card is hanging at a distance of twenty 
feet from the patient, who is able to read the No. 20 line, we 
make a record as follows: V. = When tested in this 
manner V. = is universally accepted as the normal standard, 
and yet it must be regarded only as the general average. 
Young persons under forty years, with emmetropic eyes, can 
usually see somewhat better than this, while with the advance 
of age the acuteness of vision gradually diminishes. The fol¬ 
lowing table shows the average acuteness of vision at the 
different ages: 



. 

10 years...... 

20 “ ., 

. 2 *V 


. u 


. 


. A& 

li r . T . ,.. 

. 

nn li . 

. u 

. 

80 “ . 

11 










206 


METHOD OF EXAMINATION. 


Therefore, it is not at all uncommon to meet with cases 
that can see better than -f#. Their vision then is above the 
normal standard, and its acuteness is equal to yf or sometimes 
even by which is meant that they can read letters at 
twenty feet which ordinarily can be seen only at fifteen feet or 
at ten feet; therefore, their vision is above or better than the 
standard. This should cause no surprise, as it not infre¬ 
quently happens. Many healthy young persons, whose diop¬ 
tric media and nervous elements are perfect, enjoy an acute¬ 
ness of vision greater than that which Snellen has taken as the 
standard. The acuteness of vision of Snellen, however, is not 
the maximum , but is to be taken as the average of the different 
ages. 

The maximum acuteness of vision could not serve us in 
practice, where we desire to know what is to be considered as 
normal, and the limits beyond which it must be considered as 
abnormal. But some standard must be adopted, and Snellen’s 
test-letters at twenty feet is about the greatest distance for 
normal vision in the largest number of cases. And taking this 
as the standard, we should desire in all our examinations to 
make, if possible, the person examined at least see J~£. This is 
the standard toward which we work and endeavor to reach by 
the proper correcting glasses, when we have a patient whose 
visual acuteness is below the normal. 

Among hospital patients, some are so illiterate that they 
cannot read letters. In such cases we have to use a sign, 
adopted also by Snellen, in the shape of the letter E, which is 
square, with one side open and the ends pointing in different 
directions, as upward, downward, to the right and to the left. 

EU3H 

Test-types for those who cannot read. 


This method is of service also with children and mutes. 
The card is of the same size and appearance as the ordinary 
test-card, the above characters in the various sizes taking the 


METHOD OF EXAMINATION. 


207 


place of the regular test-letters. The patient is asked in which 
direction the limbs or open part of the character points; this is 
the only distinction he can make between them. The result is 
recorded in the same way as a regular examination with test- 
types, as V. = f|, orf£, and so> on. But it has been found 
that where persons are so illiterate or children so young as to 
be unable to read the regular letters, their answers will be 
more or less unsatisfactory even with the characters above 
described, and it is consequently often a very difficult matter 
in such cases to ascertain the acuteness of vision, to> determine 
the refraction, and to decide just exactly what is the proper 
glass to give them. 

Snellen’s series of letters, although the ones most widely 
known and in most universal use, are not the only ones which 
have been constructed for the determination of the acuteness 
of vision at a distance; but none of the others have any advan¬ 
tages over Snellen’s, which are admirably suited for their pur¬ 
pose. And the name of Snellen has consequently become a 
“household word” with opticians all over the world. 

This, then, is our practical method or test for ascertaining 
the acuteness of vision, and it will be noticed that it is not only 
very simple and practical, but it is also eminently rational and 
affords us a reliable test for all errors of refraction. It also 
excludes for the greater part the exercise of the accommoda¬ 
tion, and places the retinal images of the emmetropic and ame- 
tropic eyes on the same equality as to size. 

This last advantage is a very great one, and in order that 
it may be more fully appreciated it will be necessary to con¬ 
sider the influence of correcting lenses on the acuteness of 
vision. It is a fact of common observation with which the be¬ 
ginner in optical studies is familiar, that convex lenses magnify, 
whilst concave lenses diminish the size of objects. From this 
fact it might be supposed that hypermetropes, by the aid of 
their correcting glasses (which are always convex), would re¬ 
ceive retinal images of larger size than emmetropes and my¬ 
opes; and that myopes, on the other hand, by the aid of their 
correcting lenses (which are always concave), would receive 
retinal images of smaller size than emmetropes and hyper¬ 
metropes. It would, therefore, seem at first sight as if cor- 


208 


METHOD OF EXAMINATION. 


recting lenses would completely mar the results of our deter¬ 
mination of the acuteness of vision in so far as they might 
change the fundamental condition, viz.: the equality in the size 
of the retinal images coming from the same objects. 

But then, again, it must be remembered, and it can be 
easily understood, that a hypermetropic eye, which is a flat 
eye, other things being equal, will receive (on account of the 
lack of converging or magnifying power of the eye) retinal 
images which are smaller than those received in a longer eye, 
that is, in an emmetropic or a myopic eye. The smallness of 
the hypermetropic eye and its lack of refractive power have, 
therefore, an effect on the visual acuteness and on the retinal 
image just the opposite of its correcting lens. When we ex¬ 
amine the condition of the myopic eye we find the same prin¬ 
ciple holds good in producing an opposite effect; here the 
largeness of the eye and the increase in its refractive power 
have the eff ect (within the proper limits) of increasing the visual 
acuteness and enlarging the retinal image, which stands in 
contrast to the effect of its correcting concave lens. Now the 
interesting question occurs, which of these two influences is 
the greater? Whether, in the case of the hypermetropic eye, 
the diminishing effect of the refractive condition of the eye is 
more or less counterbalanced by the magnifying effect of its 
correcting convex lens; and whether, in the case of the myopic 
eye, the magnifying effect of the refractive condition of the eye 
is more or less counterbalanced by the diminishing effect of its 
correcting concave lens? This is a question of considerable 
practical importance, whether we should expect to find in a 
hypermetropic eye wearing a convex lens a visual acuteness 
greater than that of an emmetropic eye, simply because one 
wears magnifying glasses, while the other sees only with the 
naked eye; or whether, on the other hand, we ought to expect 
to find on the part of the uncorrected hypermetrope a dimin¬ 
ished acuteness of vision corresponding to his normal retinal 
perception, because the hypermetropic eye, on account of its 
shortness, receives smaller retinal images. 

Again, the question occurs as to whether a myopic eye, 
which, with its correcting lens, reads exactly the same letters 
as the emmetropic eye at the same distance, has a natural 


METHOD OF EXAMINATION. 


209 


greater visual acuteness than the emmetropic eye, simply be¬ 
cause it is able to distinguish these letters in spite of the dimin¬ 
ishing effect of the concave lens. These questions can be 
solved only by means of minute and tedious calculations 
mathematically worked out; but as the aim of this work is 
essentially practical, it seems best to omit as much as possible 
all dry and uninteresting details, and we will therefore only 
glance at the results of the researches that have been made in 
this direction. 

The various forms of ametropia may be corrected by 
lenses of differing strengths, according as they may be placed 
at a greater or less distance from the eye. It has been deter¬ 
mined, and it is generally accepted as a fact, that if the correct¬ 
ing lens of an ametropic eye be placed in the anterior focus of 
the eye at the proper distance in front of the cornea, the retinal 
image of this ametropic eye should be of the same size as that 
formed in an emmetropic eye. 

On the other hand, if we. correct a case of hypermetropia. 
by a convex lens (stronger, perhaps, than is really necessary) 
placed nearer to the cornea, the retinal image will be smaller,, 
because the lens is not placed in a position sufficiently favora¬ 
ble for completely overcoming the diminishing effect of the 
refractive condition of the eye; while a weaker lens may be 
used if it is placed farther from the cornea than the point indi¬ 
cated, and the retinal image will be larger because the lens is 
placed in such a position as to be able to more than neutralize 
the natural refraction of the eye. 

An analogous effect is produced in the case of myopia. 
Here the diminishing effect of the lens makes itself felt in pro¬ 
portion as it is removed from the anterior focus of the eye, 
while the magnifying effect of the greater length of the eye¬ 
ball preponderates and is manifest when the correcting lens is 
approached nearer than the indicated point. These facts 
suffice to show the undoubted advantages of determining the 
visual acuteness at a distance. 

DISTANCE OF TEST-CARD. 

Now the question occurs as to what is the most favorable 
distance for determining the acuteness of vision. We deter- 


210 


METHOD OF EXAMINATION. 


mine the refraction and the visual acuteness at the same time, 
and this should be done at such a distance as to exclude the 
accommodation as much as possible. For this purpose, by 
common consent, a distance of twenty feet should be preferred, 
because rays proceeding from that distance are so little di¬ 
vergent and are so nearly parallel as to be considered practi¬ 
cally parallel, which simulates the rays proceeding from infinity 
—such parallel rays being exactly focused upon the retina of 
the emmetropic eye, without any effort on the part of the eye 
or of its accommodation; and, consequently, at this distance 
the letters which should be seen at twenty feet will form per¬ 
fect images upon the retina. 

Now while twenty feet is the standard distance, and while 
it is desirable that every optician should, if possible, always 
test his cases at that distance, yet some opticians may be so 
fixed as to make such a distance impracticable, in which cases 
a shorter distance will have to be used without very materially 
marring the results of the examination, if it is remembered 
that the rays from this shorter distance are not perfectly 
parallel, and that therefore an allowance will have to be made 
for the exercise of a very slight effort of accommodation in 
overcoming these scarcely diverging rays. 

ILLUMINATION. 

In the use of the test-letters, it should be remembered that 
the illumination of the letters is an important matter, and its 
variation may make a great difference. It should always be 
the same, with a good, clear light from a window falling on 
the card, and the patient so placed that the light will not be 
unpleasant to the eye—that is, with his back to the window, so 
that his eyes will be in the shade. 

TESTING VISUAL ACUTENESS. 

The examination is then commenced with the patient 
comfortably seated facing the card and at the proper distance; 
and with both eyes open he is asked to name the letters on 
the lowest line it is possible for him to read, and we then make 
a record of his answer in our book. If he can read every letter 
in the No. 20 line we note his vision as being equal to -g-J, or 1, 


METHOD OF EXAMINATION. 


211 


which we write V. = t|ie V. being the abbreviation for 
vision. If he reads this line with great difficulty, or is able to 
read only a portion of the letters on the line, we note the result 
as V. = f J scarcely, or V. = ?, the interrogation point sig¬ 

nifying the difficulty of reading this line or the doubt with 
which it is seen. If, however, the patient is able to read only 
the No. 30 line or the No. 40 line, we write it V. = If or }, or 
V. = |-§- or -J, in one case the vision being two-thirds of the 
normal and in the other case one-half of normal vision. 

In recording the results of the examination of the visual 
acuteness, it is best not to reduce the fractions that express 
that acuteness, but to allow them to stand as first written. If 
we write V. = -J, we know just what proportion of normal 
vision the eye possesses, but that is all the fraction expresses. 
But if we write V. = f| > we then have a fraction that not only 
expresses the acuteness of vision, but its numerator and de¬ 
nominator also have a definite meaning, the one expressing 
the distance at which the patient is seated and the other the 
size or number of the letters he is able to read. 

Having thus determined the visual acuteness of the two 
eyes together, we then test each eye separately, and again 
make a record of the result. We then write it R. V. = -§-£, 
L. V. = ff, the R. and L. standing for right and left eye 
respectively. 

After having thus found that in the emmetropic eye V. = 
, which has reference to the visual acuteness in the line of 
direct vision, or at the yellow spot, in some cases it is then 
desirable to examine the sensibility of the retina in the peri¬ 
pheral portions. 

Although in the peripheral parts of the retina V. cannot 
equal f-J, as we have already seen, on account of the dimin¬ 
ished sensibility of the retina, still it is well to know whether 
its perceptive elements are in a normal condition, and whether 
the sensibility of the retina is deficient in any of its parts. The 
power to see with the peripheral portions of the retina may be 
much diminished by certain diseased conditions of the eye. 

For this examination the most simple manner of test is for 
the optician to sit in front of the patient to be examined; the 
patient’s one eye is covered and he is directed to look at the 


212 


METHOD OF EXAMINATION. 


optician’s eye with his uncovered eye, the two persons being- 
separated by a distance of about two feet. While the patient 
keeps the optic axis of his eye directly forward, the optician 
holds his hand, or a pencil, or any bright object, in different 
positions around and in front of the eye, and as far away as 
possible. 

The farthest point at which the finger or pencil can be 
seen will show the quantitative , and the distance at which 
the fingers can be counted will show the qualitative field of 
vision. 

If the optician uses his right eye for testing his patient’s 
left eye, and the finger or pencil is held just between the eyes, 
the operator can compare the field of vision of his own eye 
with that of the eye under examination. 

Another excellent method of testing the field of vision is 
to place the person to be examined in front of a blackboard, at 


>o- 



a close distance (say about twelve inches), and, after covering 
one eye, direct the patient to look steadily with the other eye 
at a small mark on the blackboard directly opposite the eye. 
A piece of white chalk, held in the hand of the optician, is 
then to be carried along the surface of the board from its 












METHOD OF EXAMINATION. 


213 


outer edge toward the center, on a vertical or a horizontal 
line, until it can be perceived by the eye under examination 
simply as a white object, and a mark is then made at that 
point. In the same way the optician will proceed to test all 
the other meridians of the blackboard, with the same mark as 
the center, and place a dot at each point where the chalk is 
first perceived in each meridian. 

This record can then be easily transferred to a small sheet 
of paper by drawing the center dot and the various marks in 
their proper positions on the respective meridians. Then 
measure the distance in inches from the center mark outward 
on each meridian, and then a line drawn to connect each mark 
will give the size and shape of the visual field. 

An excellent instrument called the perimeter has been de¬ 
vised for accurately testing the field of vision. It consists, 
essentially, of some arrangement to keep the eye and the 
visual axis in one position throughout the examination, and 
of an arc capable of carrying a test object to all parts of the 
field of vision, and of indicating exactly the position of such 
a test object at any given time. To these necessary parts 
some perimeters add a registering apparatus; with others the 
results must be marked on the chart by the observer from the 
readings he notes on the graduated scales. For recording the 
results of tests made by the perimeter, it is best to use blank 
charts, which are printed for that purpose. 

There are a great variety of perimeters on the market, 
ranging in price from $20 to $120. One of the simplest and 
best is Emerson’s, the price of which is twenty dollars. This 
consists of a brass stand with an upright and an arm one- 
fourth of a circle. At the end of this arm there is a half circle 
of brass, which is moved on the smaller arc by a pivot in the 
center. In this there is an opening through which the person 
examined must constantly look, while the head is -steadied by 
a chin-rest, adjustable to any desired height. This semicir¬ 
cular arc has a radius of five inches, and is graduated in de¬ 
grees so that it can be placed to correspond with any of the 
meridians of the eye. The upright which extends from the 
stand for the chin to rest upon brings the eye exactly on a 
level with, and in front of, the opening in the arc. There is, 


214 


METHOD OF EXAMINATION. 


also, a slide moving freely on the arc from end to end, on 
which is placed a small disk of white paper. Then, with the 
patient in the proper position, the test is made by moving the 
slide on the arc toward the center until the disk can be seen. 
This test gives the quantitative field, while a small letter 
placed on the disk and used in the same way gives the qualita¬ 
tive field. 

This instrument is small, compact and very useful, as by 
changing the white disk of paper to one of any other color we 
can test the field for its power to distinguish colors in all the 
peripheral parts of the retina. It is a fact that the normal 
field varies in size for the different colors, that for white being 
the largest, blue next, then red, and that for green the 
smallest. 

After the extreme limits of the field of vision have been 
mapped out in this way with the perimeter, the slide, with the 
disk, should be slowly carried completely up to the center. If 
the white disk should disappear or become blurred at any 
point, there should be a careful record made of all such points 
at which the blurring commences, and also' the points where 
it becomes clear again, as it is carried toward and to the 
center. When all the meridians have been carefully examined 
and the test completed, it may be found that a certain portion 
of the retina has lost its sensibility to rays of light, though its 
functions may be perfect all around this deficient portion. In 
this manner a diagram is mapped out which reveals any spots 
of deficient vision on the retina, which are technically known 
as scotomata, or blind spots, from any cause, such as retinal 
hemorrhage, etc. 

It might be well here to add a word of caution not to mis¬ 
take the normal blind spot, or “Blind Spot of Mariotte,” for 
one of any danger. This is a small blind island or scotoma 
that is due to the optic disk or papilla at entrance of optic 
nerve; the explanation of the deficiency of vision at this point 
having been already explained in a former part of this work. 
By repeating the examination of the field of vision at different 
times, it can be ascertained whether the field is contracting or 
expanding, and of the presence or absence of any scotomata, 
as well as of their extent. 


METHOD OF EXAMINATION. 


2I& 

The normal field of vision is not circular, as a reference 
to the above diagram will prove, but extends outward about 
95°, upward about 53 0 , inward about 47 0 and downward about 
65°. The shortening of the field upward and inward is due,, 
chiefly, to the projection of the ridge of bone surrounding the 
upper part of the orbit, and to the bridge of the nose, and also,, 
to some extent, to the fact that the outer and lower parts of 
the retina have a less acuteness of perception than the upper 
and inner parts. The acuteness of vision diminishes pro¬ 
gressively toward the periphery of the field, and the farther 
from the fixation point the larger must be the objects in order 
to be distinguishable from each other. 



Diagram, Showing Outward and Inward Extent of Field of Vision. 

The above diagram serves to illustrate the projection or 
extent of the field of vision on the semicircle of the perimeter 
to its extreme temporal (95°) and its extreme nasal (47 0 ) 
boundaries, and also the portion of the retina (a to b) which 
corresponds to this extent of field. This diagram also shows 
that the most sensitive portion of the retina, or, in other 
words, that portion of the retina which is most used, extends 
further forward on the nasal than on the temporal side. This 
diagram further illustrates the remarkable fact that the field 
of vision extends on the temporal side beyond 90°. 

In the use of the perimeter the arc of the circle can be 
placed at any meridian we desire to test, as it is readily mov¬ 
able on the center, while at the apex there is a dial with a 




2l6 


METHOD OF EXAMINATION. 


pointer which marks the meridian at which the arc stands. 
Generally the examination is made in four meridians, as the 
vertical and horizontal and the two intermediate meridians, as 
at 45 0 and 135 0 ; but the number of meridians examined may 
be increased to any number the optician may think necessary 
to make the examination complete. 

It should be remembered that if there be any contraction 
of the field of vision shown by the chart it is the opposite side 
of the retina that is affected; the outer part of the field, as indi¬ 
cated on the chart, representing the inner part of the retina. 
This is well illustrated in the diagram showing the outward 
and inward extent of the field. 

The above procedures are our methods of ascertaining 
the extent of the field of vision, by which is understood the 
space throughout which the eye is able to see or distinguish ob¬ 
jects while its visual axis is directed to a certain fixed point in 
front. And as we call that direct vision which pertains to the 
yellow spot, we call that indirect vision which belongs to the 
retina outside of the yellow spot. 

This indirect vision, although it may be very indistinct 
and imperfect in comparison with central vision, is, however, 
scarcely less important. Deprived of this indirect vision, a 
man would be in the position of a person looking through a 
long, narrow tube, which would allow of his seeing nothing 
but the object to which the axis of vision is directed. It would 
be impossible for him to see objects to one side or the other 
without an incessant turning of his head. We can scarcely 
imagine the great inconvenience a person would experience 
in looking about him with such a state of vision, that is, with 
a visual field restricted to central vision. 

INDIRECT VISION. 

With such vision it would be absolutely dangerous to 
attempt to cross Chestnut street or Market street in Phila¬ 
delphia on a busy day, and it would be positive suicide to 
venture across Broadway, New York, unassisted. As a 
person walks across a street and looks straight ahead to the 
opposite side of the street where he is going, he is able, with¬ 
out in the least turning his head or eyes one way or the other, 


METHOD OF EXAMINATION. 


21 7 


to see perfectly well whether there are any carriages or cars or 
other vehicles approaching from either direction, or whether 
the street is clear of danger to pedestrians. So a man can 
walk along with the most perfect security over the most 
uneven ground and avoid obstacles that present themselves 
in his way, without once turning his eyes directly to the 
ground. So, again, a player on the piano fixes his eyes di¬ 
rectly on the music before him which he is playing, while at 
the same time, without changing the direction of his sight, he 
is able to watch the movements of his fingers over the keys. 
All this is accomplished by means of indirect vision, the 
importance of which can hardly be overestimated, and it 
would not be possible without a healthy condition of the 
various portions of the retina. If a person would attempt to 
cross the street with a shade extending forward from each 
temple, or would try to walk over uneven ground or play the 
piano with a screen placed horizontally below the eyes, he 
would meet with such difficulties as to be speedily convinced 
of the very important service indirect vision renders mankind 
in the ordinary occupations of every-day life. 

The movements which the leader of an orchestra makes 
with his baton might at first sight to the casual observer seem 
to be superfluous. But from the above illustrations it can be 
easily understood that the members of the orchestra, although 
they cannot turn their eyes from their notes, are able to per¬ 
ceive by means of their indirect vision every movement of 
their leader. It would be easy to multiply examples of the 
value of indirect vision, since all our occupations and all our 
movements in the streets and in our homes would be difficult 
in the extreme if we did not possess this important faculty of 
vision. It is this, in a word, which enables mankind to avoid 
those things which approach or menace him from all sides. 

PERCEPTION OF LIGHT AND COLORS AND LETTERS. 

The perception of light remains almost exactly the same 
throughout the whole extent of the retina, but there is a wide 
difference when it comes to the perception of colors and the 
visual acuteness. The perception of colors is much less acute 
in the eccentric portions of the retina than at or near the 


218 


METHOD OF EXAMINATION. 


yellow spot, and diminishes progressively in proportion as it 
departs from this central point, while the visual acuteness 
diminishes still more rapidly as the periphery is approached. 

In order to illustrate the relation these functions bear to 
each other, we will take, for example, the standard of meas¬ 
urement for all three functions at the central point of vision to 
be i. The perception of light at a point 30° from the center 
would still be 1, while the perception of colors would be 
and the visual acuteness only -fa . These figures, of course, 
have only relative value, but they serve to establish the fact 
that of the three functions the perception of light remains the 
most constant, while the perception of colors and the visual 
acuteness diminish rapidly toward the periphery of the retina, 
the latter more than the former. 

These facts are of importance in explanation of these 
different functions of the retina. It proves conclusively that 
these three functions are entirely distinct from each other, 
and cannot be reduced to a single one, as might be thought 
possible. 

After the optician has learned how to examine the limits 
and functions of the different portions of the retina as shown 
in this indirect vision, according to the methods just de¬ 
scribed, it remains to make a practical application of this 
knowledge to the numerous diseased conditions of the retina 
in which indirect vision is likely to be found affected, as there 
is scarcely a lesion of the interior of the eye that is not accom¬ 
panied by symptoms that can be detected by the perimeter; 
besides which all the diseases of the brain and spinal cord 
which are made manifest by eye symptoms, commence by 
some abnormality in the form and functions of the visual field. 
But this leads into the consideration of matters of organic dis¬ 
ease, which are outside the province of the optician and there¬ 
fore beyond the scope of this book. However, it may be well 
to take up enough space to briefly mention some of the dis¬ 
eases in which the application of perimetry is of advantage. 

In glaucoma one of the first symptoms of the disease is a 
restriction of the visual field. In retinal hemorrhages com¬ 
plete abolition of vision is found in the parts affected, which is 
manifested by a fixed scotoma in the visual field. In a like 


METHOD OF EXAMINATION. 


219 


manner is the field affected by inflammatory affections of the 
choroid; and in fact there is no affection of the retina whose 
progress cannot be traced by the disturbances caused in the 
field of vision. 

DIRECT VISION. 

After this digression in considering indirect vision, we 
now return to the examination of direct vision, which has 
been shown in the emmetropic eye to be equal to f-jf, but 
which, as has already been stated, is only the average sight of 
all ages, and not by any means the maximum degree of sight. 

Each eye must he tested separately, by placing the black 
metal disk in the trial-frame over the eye which is not being 
tested, and then changing it when the other eye is to be exam¬ 
ined. After the optician has found that his patient’s vision is 
, it will enable him to decide several points as to the exist¬ 
ence or non-existence of refractive errors. In the first place, 
he can exclude myopia, for this defect always impairs the 
visual acuteness, and it is not possible for even the slightest 
degree of myopia to exist with an acuteness of vision of f J. 
In the second place, he can reasonably exclude astigmatism, 
for this defect also mars the acuteness of vision to a greater or 
less extent, although it is possible with a vision of J-j- for a 
slight degree of astigmatism to be present, especially if it be 
hypermetropic astigmatism, which the accommodation is able 
to overcome and conceal. In the third place, when the opti¬ 
cian finds a vision of he must not hastily jump at the con¬ 
clusion that his patient is emmetropic, for a large degree of 
hypermetropia may be (and often is) present, but is so 
concealed by the tension of the accommodation as to not in 
the least interfere with perfect vision at twenty feet, and the 
sight will apparently be as good as that of any emmetrope. 
Hence, the next step is to determine whether or not hyper¬ 
metropia is present, and if so, what is the amount of the mani¬ 
fest hypermetropia. 

TESTING FOR HYPERMETROPIA. 

For this purpose the optician places before his patient’s 
eye a weak convex lens, say about + .50 D., while the patient 
looks at the lowest line he is able to read on the distance test- 


220 


METHOD OF EXAMINATION. 


card hanging twenty feet away, which in this case is No. 20. 
If the letters are dimmed and the sight made worse, then a 
weaker lens (+ .25) is tried; if the result of this is again to blur 
the letters, the optician may reasonably conclude that the 
patient has no hypermetropia, at least no manifest hyper¬ 
metropia, and the case is set down as one in which the refrac¬ 
tion of the eye is emmetropic. 

If, on the other hand, with the convex lenses of .25 D. or 
.50 D. the sight remains just as good as it was before, or if it 
is made better, then the case on hand is proven to be one of 
hypermetropia, and the optician proceeds to determine the 
degree of the defect by trying a + .75 and a + 1, and keeps on 
trying still stronger lenses. 

In ascertaining the degree of hypermetropia in any case 
under examination, the optician places before the eye stronger 
and stronger convex lenses until he reaches a lens that begins 
to blur the letters slightly and with which the patient is not 
able to read the No. 20 line so clearly. The optician should 
then make a note in his record book of the strongest convex 
lens that improves sight, or the strongest convex lens through 
which sight is as good as without any lens, which lens will 
represent the amount of the manifest hypermetropia. It 
should be written H. m. (which is the abbreviation for mani¬ 
fest hypermetropia) = + .50 D., or H. m. = + 1 D., as the 
case may be. This is the examination of a case, then, in 
which, although the acuteness of vision is normal, the refrac¬ 
tion of the eye is abnormal, being hypermetropic. 

If, however, the test of the patient’s vision falls below the 
normal standard, that is, if at twenty feet he is able to read 
only the No. 30 line, or No. 40, or No. 70, his vision being -f£, 

or f^r, the question then arises: “What is the optical 
defect present that causes the impairment of vision; is it hyper¬ 
metropia, or myopia, or astigmatism, or amblyopia?” 

By following the method just outlined above, the optician 
is able at once to determine whether or not it is a case of 
hypermetropia. 

TESTING FOR MYOPIA. 

Having, then, excluded hypermetropia as one of the pos¬ 
sible causes of the impaired vision, the optician should next 


METHOD OF EXAMINATION. 


221 


proceed to determine whether myopia is present or not; and 
this is done by testing the eyes with concave lenses. He 
'should commence with a weak concave lens, — .50 D., or even 
in some cases a — .25 D., and if this lens causes a marked 
improvement in sight, it proves to the optician’s mind the 
presence of myopia. He then tries successively stronger and 
stronger concave lenses, at intervals between them of .50 D., 
until he reaches a lens that raises vision to The weakest 

concave lens that enables the patient to read the No. 20 line at 
twenty feet will be the measure of the myopia. In cases of 
high myopia it is impossible, in the majority of the cases, to 
find any lens that will bring the vision up to and in such a 
case the weakest concave lens that affords the best vision will 
be the measure of the myopia. In the case under examina¬ 
tion with concave lenses, the refraction of the eye is proven to 
be myopic. 

AN ILLUSTRATIVE CASE. 

A young woman, about twenty years of age, comes to an 
optician, complaining of asthenopic symptoms, that is, her 
eyes soon get tired and give out after she has used them for 
some time, and this is especially the case in the evening. She 
looks to be anaemic and in poor health. The bridge of the 
nose is flat, as are also the parts around the angle of the jaw. 
There is slight asymmetry of the face, the left half being 
apparently more and better developed than the right. No 
muscular insufficiency; the pupils are equally movable in each 
eye. The patient says the left eye is the best, a fact which 
seems to be corroborated by the appearance of the face. 

The optician, therefore, examines this eye first, and finds 
the acuteness of vision to be equal to J-J. This is normal 
vision, and excludes myopia or astigmatism; but still there is 
a possibility of the existence of hypermetropia, either manifest 
or in a latent form, that is, concealed by the action of the ac¬ 
commodation. Weak convex lenses are tried and are rejected; 
this proves at least that there is no manifest hypermetropia, 
but leaves open the question of the existence of latent hyper¬ 
metropia. The record is made L. E. V. = J-J. 

The right eye is then examined, and the acuteness of 
vision found to be only T W A trial with convex lenses results 


222 


METHOD OF EXAMINATION. 


as follows: + i D. makes the same line of letters clearer and 
plainer, and some letters visible in the next line. This im¬ 
provement continues with stronger lenses, until it is found' 
that + 3 D. makes vision equal to f-J. This seems to be the 
correction for this eye; as + 3.50 makes vision worse. The 
record to be made in the case-book would read as follows: 
R. E. V. = t 2 o°o> H. m. = + 3 D., with which V. = f$. 

The youth of the patient indicates the probability of the 
existence of some latent hypermetropia in the left eye as well 
as in the right. This presumption is justified by the fact that 
both eyes are soon fatigued by close use, which is nearly 
always the case in hypermetropia. The optician then wants 
to determine whether or not there is any latent hyperme¬ 
tropia, and he asks, how can it be done? There are two in¬ 
fallible means—the use of atropine and of the ophthalmoscope. 
There are innumerable objections to the use of atropine by 
the optician, and the use of the ophthalmoscope is not suffi¬ 
ciently familiar to the majority of opticians to make it avail¬ 
able in the present case. 

In the absence of the information afforded by either of 
the above means, the optician will look to the near point and 
the amplitude of accommodation as being able to throw some 
light on the question. A person twenty years of age pos¬ 
sesses an amplitude of accommodation of 10 D. and a near 
point of four inches. A hypermetrope of the same age will not 
possess the same amplitude of accommodation, and conse¬ 
quently will not be able to see as near. The presence of 1 D. 
of hypermetropia will diminish the amplitude of accommoda¬ 
tion by that amount (10 D. — 1 D. = 9 D.) and will make the 
near point about four and a half inches. A hypermetropia of 
2 D. means an amplitude of accommodation of 8 D. and a 
near point of five inches. A hypermetropic eye of 3 D. pos¬ 
sesses only 7 D. of positive refraction, with the near point at 
five and a half inches. 

Bearing these points in mind, the optician is able to use 
them to approximate the degree of ametropia present. In 
this case, the examination of the right eye shows the near 
point to be situated at six and a half inches, which indicates a 
hypermetropia of -f 4 D.; and as the previous examination 


METHOD OF EXAMINATION. 


223 


with test-lenses showed a manifest hypermetropia of 3 D., 
there remains a latent hypermetropia of 1 D. The near point 
of the left eye is found to be at four and a half inches, which 
means a loss of refractive power of 1 D., or a latent hyper¬ 
metropia of that amount. 

In this case it would be proper to order R. + 3 D., L. + 
.50 D. 

And the use of these glasses would most probably cause 
all the symptoms of asthenopia to gradually disappear. It will 
be noticed that the glasses are not ordered sufficiently strong 
to entirely correct the total hypermetropia, because young 
persons usually possess such an excess of accommodation as 
to prevent its relaxation sufficiently to admit of a full correc¬ 
tion being made. 

The question occurs as to what to attribute the impair¬ 
ment of vision in the right eye, in spite of the correction of its 
hypermetropia. It is not due to the small size of the retinal 
images, because after the correction of the defect by glasses 
they are of the same size as those of an emmetropic eye. It 
is more probably due partly to a lack of development of the 
retina, and partly to a lack of'use of the eye. The condition 
of a hypermetropic eye is essentially due to a lack of develop¬ 
ment. of the organ, which affects the retina in such a way as to 
make it less sensitive. This places the right eye of this 
patient in a less favorable condition for use than the left, and 
therefore the individual has naturally preferred to use the 
better eye to the greater or less exclusion of the other, per¬ 
haps involuntarily without his own consciousness, the con¬ 
tinuation of which only intensifies and perpetuates the trouble. 

It is of great importance to this patient to cause her to 
bring her right eye into active exercise. She should, there¬ 
fore, be advised to read or work half an hour twice each day 
with the right eye alone by the aid of her correcting lens, and 
at the same time the other eye should be closed. 

TESTING FOR ASTIGMATISM. 

The above practical case was introduced to illustrate the 
mode of procedure in the examination of an every-day case, 
and we now pass on to outline the further steps in the process 


224 


METHOD OF EXAMINATION. 


of examination of a case of optical defect. If concave glasses 
do not improve vision, or improve it but slightly, it is proven 
to be not a case of myopia. We now have a case in which we 
have excluded both hypermetropia and myopia, and in further 
hunting for the cause of the impaired vision our next thought 
will be as to the existence of astigmatism. 

To determine the existence of this defect, the patient’s 
attention is directed to the astigmatic cards hanging on the 
wall, either the card of Pray’s letters or the card of radiating 
lines. If some of the letters or some of the lines appear very 
much blacker and clearer than others, it is proof of the exist¬ 
ence of astigmatism, and the optician proceeds to further ex¬ 
amine the case to determine its nature and degree, by follow¬ 
ing the directions which will be given in the chapter on 
astigmatism. 

The optician now has examined his case for hypermetro¬ 
pia, myopia and astigmatism, and if either of these defects has 
been present, he has been able to detect its existence by fol¬ 
lowing the above methods. 

Sometimes in his work the optician will meet with a case 
in which neither of these defects exists, and in which he is un¬ 
able to improve the vision by any glass or any combination of 
glasses. This, then, is probably a case of amblyopia from 
some cause; and here comes in the value of the use of the 
ophthalmoscope to determine the seat and cause of the im¬ 
paired vision. It is most probably located either in the cornea, 
as spots, or deposits, or opacities of this membrane; or on or 
in the crystalline lens, incipient cataract; or in the retina or 
optic nerve, as retinitis, or neuritis, or atrophy. With a little 
practice and experience with the ophthalmoscope the optician 
is able to recognize which of these conditions is present, and is 
able to advise his patient intelligently as to what course to 
pursue in seeking medical advice. Or, if the optician does 
not possess an ophthalmoscope, he is able, by other means, to 
determine whether the defective vision is due to a refractive 
error or not, by following the methods above described; and 
if not, he would be justified in advising the patient to consult 
an oculist, as the case would then be one that needed medical 
attention. 


METHOD OF EXAMINATION. 


225 


In testing distant vision, if the patient is unable to read 
even the largest letters at the top of the card when he is seated 
at the regular distance of twenty feet, he is asked by the opti¬ 
cian to arise and approach the card and to stop just as soon as 
the largest letter becomes legible. If he stops at fifteen feet, 
and says he can see the top letter on the card, which is num¬ 
bered 200, then his vision is recorded by the optician as 
If he must go up to ten feet or five feet before he is able to dis¬ 
tinguish this top letter, his vision would be or re¬ 

spectively. In some cases of high myopia the patient is com¬ 
pelled to approach as close as two feet before he is able to 
distinguish the large letter at the top of the card, in which 
case the record would read V. = . 

When the patient’s sight is less than this, that is, when he 
is unable to read any letters at any distance, then the ability to 
count fingers is used as affording a sufficient test for all prac¬ 
tical purposes. One, two or three fingers are held between 
the eye and the light, and the greatest distance at which they 
can be counted is observed and recorded in the optician’s 
record-book. When fingers can no longer be counted, it is 
not usual to speak of vision, but only of perception of light; 
and this is distinguished as qualitative or quantitative. 

The patient with qualitative perception of light will see 
and distinguish the outlines of any bright object, such as a 
sheet of white paper, when it is presented to him at a favor¬ 
ite angle, and will be able to recognize large, dark marks 
upon it, or, perhaps, can detect the difference between the 
white margin and the printed portion of the page. The 
patient with quantitative perception of light will be able to 
discern the lighter from the darker parts of the room, or, at 
least, will be able to point out the position of a flame, and will 
know when it is lowered or concealed. When perception of 
light is lost, the eye may be considered as beyond the reach of 
art, except in some very rare cases of glaucoma, where a 
timely operation will sometimes do wonders in restoring 
vision that is apparently hopelessly lost. In making a note 
in his record-book of those cases in which only perception of 
light exists, it is customary for the optician to write it as p. 1. 


226 


METHOD OF EXAMINATION. 


Several points in the examination of distant vision are of 
sufficient importance to require emphasis and reiteration. 

The card of test-letters should be hung in such a position 
as to receive the best possible illumination, and at such a dis¬ 
tance (twenty feet, if possible) as to exclude the need of 
accommodation. 

Each eye should be tested separately, commencing, 
usually, with the right eye, or, in cases of marked anisome¬ 
tropia, with the eye that possesses the best vision, while the 
other eye is covered by an opaque metal disk placed in the 
trial-frame. This is a much better method to exclude the 
other eye than to allow the patient to close his lids, as he will 
either not close them completely, or else he will close them so 
tightly as to also unconsciously shghtly contract the muscles 
of the eye under examination, in either of which cases the 
perfect result of the examination may be changed. Neither 
should he be permitted to cover the eye with his hand, as he 
may either look through his fingers or he may press on the 
eye so tightly as to momentarily interfere with the function of 
the retina, and thus again, in either case, vitiate the result of 
the examination. 

In determining the refraction of an eye, the test of the 
distant vision with trial-lenses should always commence with 
convex. 

After the refraction of each eye has been separately ascer¬ 
tained, the two eyes are then tried together, when it will be 
found that binocular vision (which means single vision with 
two eyes) is much better and more satisfactory than that of 
either eye separately. And this is even the case in persons of 
marked anisometropia (each eye differing in vision and in re¬ 
fraction) ; the vision of the best eye alone is not as good as the 
two eyes together, or, in other words, the vision of the good 
eye is improved by that of the poorer eye, strange as this may 
seem at first. It may be well to mention that there are some 
exceptional cases where the sight of the good eye is made 
worse by that of the poorer eye, and where vision is more 
satisfactory when the deficient eye is excluded. 


METHOD OF EXAMINATION. 227 

THE OPHTHALMOSCOPE AS A METHOD OF EXAMINATION. 

Ophthalmoscopy is at the same time one of the most 
important methods of examination, and one of the most diffi¬ 
cult, for a clear view of the fundus can sometimes be ob¬ 
tained only by a skillful observer. It is not only of the 
greatest importance in the diagnosis of diseased conditions, 
but as a method of determination of the various forms of 
-ametropia it is not to be despised. 

The first step for a beginner is to familiarize himself with 
the appearance of the normal fundus, a very good repre¬ 
sentation of which is given in the colored plate. This is 
necessary, in order to be able to recognize a diseased con¬ 
dition when it is met with, and even after the optician has de¬ 
veloped into an accomplished ophthalmoscopist, and able to 
easily detect morbid changes, there may still be considerable 
•difficulty to interpret the significance of these diseased ap¬ 
pearances. 

Practice and experience are the best teachers, but the 
•optician will be materially assisted in mastering this method 
by a careful study of the writings on this subject, together 
with colored plates of the fundus. 

The ophthalmoscope was given to the scientific world by 
Helmholtz, in 1851, prior to which time the interior of the 
-eve was as a closed and sealed book, and black as the proverb¬ 
ial “ace of spades.” This, to the inquiring mind, raises the 
ouestion as to why the interior of the eye is so dark, and why 
the pupil is black. 

WHY DOES THE PUPIL APPEAR BLACK? 

This may seem like a very simple question, and yet its 
answer depends upon a line of scientific reasoning that 
brought about the invention of the ophthalmoscope. The 
pupil of the eye appears black, because there are no rays of 
light passing from the eye of the observer to illuminate it. 
If a candle is held in such a way as to light up the pupil, the 
rays of light return to the candle, which is the source of light. 
If the observer attempts to intercept these return rays of 


228 


METHOD OF EXAMINATION. 


light, and thus obtain a view of the interior of the eye, as 
soon as he undertakes to place his own eye in the path of 
these return rays, he at once shuts off the source of light and 
there are then no rays to return. 

If now, by some means, the observer is able to send the 
rays of light from his own eye into the patient's eye, he will 
receive some of these rays back again, and then the pupil, in¬ 
stead of appearing black, will give a red reflex. The simplest 
manner in which this can be accomplished is by means of a 
piece of plate glass and a light placed at the side of the pa¬ 
tient’s head; by holding this glass up to his own eye the 
optician can reflect some light into the eye under observation, 
when the rays will return to the glass and pass through it 
into the observer’s eye and afford a view of that part of the 
fundus from which these rays come. 

Helmholtz improved on this bv using three plates of 
glass, which might be called the original ophthalmoscope. 
In our present improved instrument the concave mirror 
throws many more rays of light into the eye, and the per¬ 
foration allows a larger number of return rays to enter the 
observer’s eye, and hence a better and brighter image of the 
fundus is secured. 

In the use of the ophthalmoscope two methods of ex¬ 
amination are employed: the direct and the indirect. 

DIRECT METHOD OF EXAMINATION. 

The room should be darkened, and the light placed on 
the same level as the eye that is to be examined, and on the 
same side of the head, and some few inches back of it. The 
optician sits by the side of his patient and facing him, using 
his right eye to examine patient’s right eye, and vice versa. 

The pupil is then illuminated by the concave mirror of 
the ophthalmoscope, held ten or twelve inches away, when 
the red reflex is obtained, and if there are any opacities in the 
cornea, crystalline lens or vitreous, they will become dis¬ 
tinctly visible as black spots on a red background. The in¬ 
strument is then moved slowly toward the patient, all the 
while keeping the pupil well illuminated, until the mirror is 



ILLUSTRATION FROM TIFFANY’S ANOMALIES OF REFRACTION, 
BY THE HUDSON-KIMBERLY PUBLISHING CO., KANSAS CITY, MO. 













































METHOD OF EXAMINATION. 


229 


as close as spectacles can be worn and the foreheads of opti¬ 
cian and patient are almost in contact, when the retina and 
optic nerve and the blood-vessels come into view. 

This method of examination of the fundus of an eye 
allows but a small portion to be seen at one time, but as it is 
magnified fifteen diameters the minutest details are percepti¬ 
ble. The accommodation of both optician and patient should 
be relaxed, and then, if both eyes are emmetropic, a perfect 
picture of the retina of the observed eye will be formed on the 
retina of the observer’s eye. 


WHAT TO LOOK FOR. 

The red color of the fundus is due largely to the blood¬ 
vessels of the choroid. The color varies in different persons, 
and in different parts of the retina in the same individual, 
fieing most pronounced at the posterior portion. The fundus 
is darker in brunettes than in blondes, on account of the pre¬ 
ponderance of pigment matter in the former. The deeper 
color at the posterior portion of the eye is most noticeable in 
the region of the yellow spot and optic disk, and around the 
latter is oftentimes seen a black ring, known as the choroidal 
ring. 

The optic disk, that is, the entrance of the optic nerve, is 
the chief landmark looked for in the examination of the 
fundus, presenting a delicate grayish-red tint, the nasal side 
being of a little deeper color than the temporal. The disk is 
not always perfectly circular, but may be slightly oval, elon¬ 
gated either horizontally or vertically. The yellow spot is 
situated at the same level, and a little to the temporal side of 
the disk. 

The retina receives its blood supply from the arteria 
centralis retinae, which enters the eye in the optic nerve and 
divides into branches that spread out over the whole fundus 
and carry the blood to every portion of it. These arterial 
branches all have their accompanying veins, which collect the 
blood and converge into one large vein which enters the optic 
nerve near where the artery emerges. The arteries are lighter 


230 


METHOD OF EXAMINATION. 


in color than the veins, with a light band or reflex along their 
center. The veins are larger and darker and somewhat more 
tortuous than the arteries. 


DETERMINATION OF MYOPIA. 

In myopia the retina is situated too far back, and the 
rays of light proceeding from it are convergent, and will focus 
at the location of the far point. Such rays cannot be focused 
on the retina of an emmetropic observer, and hence the pic¬ 
ture of the fundus received will be blurred and indistinct, the 
disk appearing larger than normal. A suitable concave lens 
rotated into the sight hole of the ophthalmoscope will render 
the convergent rays parallel and clear up the retinal pic¬ 
ture. The amount of the myopia will be determined by the 
weakest concave lens which affords a distinct view of the fun¬ 
dus. It would be very easy for the optician to see with a 
stronger concave lens, by allowing his accommodation to 
come into play, which it has a constant tendency to do, but 
this would interfere with the accurate measurement of the 
myopia. 


DETERMINATION OF HYPERMETROPIA. 

The rays emerging from an hypermetropic eye are diver¬ 
gent, and cannot be focused upon the retina of the observer’s 
eye, except by an effort of his accommodation, or by the 
intervention of a convex lens. If the defect be not of too 
high a degree there is every incentive for the accommodation 
of the observer to exert itself and correct the defect, and make 
the retinal picture distinct; but it would be impossible in this 
way to measure the amount of accommodation used, and 
hence no indication of the degree of hypermetropia would be 
afforded. But the accommodation must be relaxed and a 
convex lens found which will render the diverging rays par¬ 
allel. When an emmetropic optician is examining ophthal- 
moscopically an eye that is hypermetropic to the extent of 
3 D., he must rotate a convex lens of the same number into 
the aperture of his instrument, which will afford a clear view 
of the fundus and represent the degree of hypermetropia. 


METHOD OF EXAMINATION. 


23I 


The temporal side of the disk is the easiest part to bring" 
into focus for the beginner, and can be used for these exam¬ 
inations, together with the small blood-vessels as they pass 
from it to the yellow spot. 

DETERMINATION OF ASTIGMATISM. 

In the use of the ophthalmoscope astigmatism may be 
suspected when the appearance of the fundus is more or 
less blurred, and when spherical lenses fail to entirely clear 
it up; besides which the disk is no longer round, but appears 
oval. In this defect the refraction of the several meridians 
of the eye is determined by the use of convex and concave 
spherical lenses focusing the blood-vessels that run in the 
meridians at right angles. 

For instance, the strongest convex or weakest concave 
lens is found that gives the clearest image of the blood-vessels 
running horizontally from the disk to the yellow spot, and 
this lens will indicate the refraction of the vertical meridian 
of the eye. In like manner the lens is selected that is required 
for the vessels passing vertically upward and downward 
from the disk, which will indicate the refraction of the hori¬ 
zontal meridian. With the refraction of the two chief me¬ 
ridians known, it is a simple matter to estimate the character 
and degree of astigmatism present. The refraction of the 
oblique meridians can be determined in the same way. 

THE INDIRECT METHOD OF EXAMINATION. 

In this method the ophthalmoscope is held at ordinary 
reading distance (twelve to fifteen inches), and the fundus is 
viewed through a strong convex lens placed within its focal 
length from the eye, and held in position by the thumb and 
index finger. The rays of light proceeding from the patient’s 
eye pass through this convex lens and are refracted by it, 
forming a real inverted image of the fundus in the air in front 
of the lens, and the observer, looking through the aperture of 
his ophthalmoscope, gets a clear view of this image. It is 
important that the convex lens be placed within the focal dis¬ 
tance of the lens. 


232 


METHOD OF EXAMINATION. 


The advantages of the indirect method may be men¬ 
tioned as follows: 

1. The optician is farther away from his patient, which 
makes the examination easier, and is of obvious advantage 
in other ways. 

2. A larger extent of surface of the fundus can be seen 
at one time. 

3. There is no necessity for the employment of correct¬ 
ing lenses in the ophthalmoscope. 

4. It is possible to obtain a view of the fundus through a 
smaller pupil. 

The advantages of the direct method are that the image 
is erect and the details of the fundus very much more mag¬ 
nified, and consequently any departure from the normal con¬ 
dition can be more easily detected. 


RET1NOSCOPY, OR SHADOW TEST. 

This method of estimating the refraction of an eye is 
especially useful in the examination of children, or of other 
persons, on whose answers entire reliance cannot be placed, 
and it is always valuable as an auxiliary test in any case. 
While it seems a little difficult at first sight, it becomes fairly 
easy after a little practice, and on account of the rapidity and 
accuracy with which it can be performed, it is regarded as 
the best objective test at the command of the optician. 

The examination is conducted something after the same 
manner as the indirect method of the ophthalmoscope, with¬ 
out the interposition of any convex lens. The light should 
be directly over the patient’s head, and the optician is seated 
at a distance of one meter, or a little more. The mirror used 
may be either plane or concave, according to the fancy of the 
observer. The light is reflected into the eye, when the red 
reflex is at once noticed in the pupil. As the mirror is ro¬ 
tated in various directions the light passes off and is followed 


METHOD OF EXAMINATION. 233 

by a shadow, the direction of the movement of which deter¬ 
mines the kind of refraction that is present. 

When a plane mirror is used, and the shadow travels 
•across the pupil in the same direction as the mirror is rotated, 
the indications are that the case is either one of emmetropia 
or hypermetropia. Convex lenses are then used in increasing 
strength until one is reached that reverses the movement of 
the shadow. In emmetropia a very weak lens will accom¬ 
plish this. 1 

If the plane mirror is used, and the shadow moves across 
the pupillary space in the opposite direction to which the mir¬ 
ror is rotated, the existence of myopia is indicated, when con¬ 
cave lenses of increasing strength are used until the move¬ 
ment of the shadow is reversed, and the lens thus found will 
he the measure of the defect. 

The vertical and horizontal meridians can be examined 
in this way, and if they are both found to be alike, the case is 
one of emmetropia, hypermetropia, or myopia; but if there 
is a difference in the refraction of the two meridians, astig¬ 
matism is present, the degree of which is represented by the 
amount of that difference. 

With a concave mirror the movements are just the reverse 
of those given above, that is, in emmetropia and hyperme¬ 
tropia the shadow travels against the rotation of the mirror, 
and in myopia zvith it. 

A dilated pupil is of advantage in retinoscopy, as is also 
a relaxation of the accommodation. If the pupil is small it 
greatly increases the difficulty of seeing the shadow. 


AN ILLUSTRATIVE CASE. 

* 

A mother brings her young daughter to the optician, 
with the statement that she cannot read or see close at hand, 
although her distant vision is as good as ever. She is about 
fifteen years of age, and is pale and weak, and her mother says 
she is just recovering from a severe attack of diphtheria, and 
is now gaining in strength every day. Previous to her illness 
her sight had always been fairly good, and this impairment of 


234 


METHOD OF EXAMINATION. 


near vision has only been noticed since her recovery from the 
diphtheria. 

This sudden appearance of difficulty in near vision re¬ 
minds the optician of the action of atropine, while the pres¬ 
ervation of normal vision at a distance is another point of 
similarity to the effect of this drug. Now it is well known 
that atropine acts by paralyzing the accommodation, and 
hence the optician is justified in presuming that the case in 
hand is one of paralysis of the accommodation. The good 
distant vision proves that neither myopia nor amblyopia is 
present. 

When a young person of this age complains of inability 
to read as close as eight inches, the optician should first ascer¬ 
tain the acuteness of vision; if this is markedly impaired, lie 
would think of the possible existence of myopia or amblyopia. 
The former can be determined by trial with the concave lenses, 
and the latter by the pin-hole test. 

But in the present case the distant vision is good, and the 
question to be solved is, “What is the cause of the impairment 
of near vision?” It is due to one of two causes—either 
paralysis of the accommodation or hypermetropia. In the 
latter case the defect may be of such a degree that all the 
power of the accommodation is needed to correct the refrac¬ 
tion and afford satisfactory distant vision, and there is not suf¬ 
ficient power of accommodation left to bring the rays from 
near objects to a focus on the retina. But, again, it should be- 
remembered that if an individual was so strongly hyperme¬ 
tropic, there would be some impairment of sight, and distant 
vision would no longer be perfect. By this manner of reason¬ 
ing, and by a process of exclusion, the optician can scarcely 
come to any other conclusion than that the present case is one 
bf paralysis of the accommodation. 

Paralysis of the accommodation in a youth is the same 
condition as presbyopia in an old person; in the one case it 
is due to disease, in the other to age. A man fifty years of 
age, whose distant vision is good, but who finds difficulty in 
reading and writing, is probably an emmetrope, over whom 
presbyopia is just beginning to steal. There may be a very 
slight degree of hypermetropia present, but the supposition of 


METHOD OF EXAMINATION. 


235 


hypermetropia becomes less probable the older the individual 
grows without the need of glasses for reading. Hypermetro¬ 
pia often shows itself as an early presbyopia, and a hyperme- 
trope will begin to feel the need of glasses at forty, or soon 
thereafter; so that in a man who has reached the age of fifty 
without wearing glasses, the presence of hypermetropia can 
almost certainly be excluded. An individual, on the other 
hand, in whom both distant and near vision are greatly im¬ 
paired (and in the absence of amblyopia) is most likely hyper¬ 
metropic, and, in a degree, the stronger as the patient is 
younger and the far point is further removed. 

Patients who say they can see well to read, but cannot see 
well at a distance, are probably myopic, while those who can¬ 
not see well, either near or far, either with or without ordinary 
spherical glasses, are probably astigmatic. 

In the case at hand there is another point that will help 
toward making up the diagnosis, and that is the condition of 
the pupils, the contraction and dilatation of which go hand in 
hand with the use or rest of the accommodation. In the 
young lady under examination the pupils are very much 
dilated. On exposure to light they contract very slowly, and 
on alternately covering and uncovering the eyes it is seen that 
they react very slowly and imperfectly to the light. 

It may now be regarded as certain that the diagnosis pre¬ 
viously made is-correct, and it remains to determine the degree 
of the affection; that is, whether the paralysis of the accom¬ 
modation is partial or complete. 

In testing the patient, the optician finds the vision without 
glasses to equal He then proceeds to make a trial with 

convex lenses, and he finds she can still read the No. 15 line 
with + 3 D., but not with + 3.50 D.; we have therefore 
H. m. = + 3 D., V. = ff. The partial correction of the 
defect by means of the accommodation is incomplete, as the 
patient is able to read only the No. 20 line. In the eyes of 
hypermetropes, where the rays are not sharply focused on the 
retina, there occur instead circles of diffusion. These are not 
sufficiently great to render the No. 20 line illegible, but they 
do make more diffuse the smaller characters of the No. 15 
line. 


236 


METHOD OF EXAMINATION. 


The optician tries the feeblest convex lens which affords 
the patient her greatest acuteness of vision, -yf, which is found 
to be + i D. She had previously read the same line with + 3 
D., and her amplitude of accommodation is therefore the 
difference between the two = 2 D., instead of 12 D., which is 
the normal amplitude of accommodation for a person of her 
age, which means a loss of 10 D. 

In determining the glass to be prescribed for reading and 
writing at twelve inches, the optician will order the weakest 
convex lens that enables her to read at that distance; he 
chooses (the weakest lens so as to require the accommodation 
to do part of the work and thus keep it in continual exercise. 

To see at twelve inches requires a positive refraction of 3 
D. (twelve inches = 30 C. M. = 3 D. A hypermetrope 
of 3 D. without accommodation would require the amount of 
his defect added to this: 3 D. + 3 D. — + 6 D. But as this 
patient still possesses 2 D. of accommodation, it should be 
subtracted from the 6 D., which leaves + 4 D. as the proper 
lens to be prescribed for reading at twelve inches. 

ANOTHER ILLUSTRATIVE CASE. 

The next patient that may apply to our embryo optician is 
an old gentleman seventy years of age. We will listen to the 
old man’s story, and give our friend some assistance in fitting 
the case. The complaint is that the glasses that have been 
used for so many years with comfort are no longer satisfac¬ 
tory. On questioning the patient, we find that he did not 
begin the use of glasses for near work until he was fifty-five 
years old. This at once raises the suspicion in our friend’s 
mind that his patient may have been somewhat myopic, for he 
knows that in emmetropes presbyopia (and with it the need of 
glasses) usually steals over the patient about the age of forty- 
five. He also knows that in hypermetropia it appears earlier, 
and in myopia later. 

The first step in the examination of any case is to deter¬ 
mine the refraction and ascertain the acuteness of vision; and 
our friend therefore asks his patient to be seated at the proper 
distance and to look at the test-card hanging on the wall, and 


METHOD OF EXAMINATION. 


23 7 


to tell him which line he is able to read, as he first excludes, 
one eye and then the other. With the right eye he distin¬ 
guishes only the No. 80 line, and with the left eye the No. 25; 
line; and as the card is hanging at a distance of twenty feet,, 
our friend writes the record in his book as follows: R. E. V.. 
= J-J, L. E. V. = -Jf. The vision of the right eye is markedly 
deficient, and on first thought it would seem as if the vision of 
the left eye was also below the standard. But our optician 
remembers what he has read in a former chapter of The Opti¬ 
cian’s Manual , that the visual acuteness gradually diminishes 
with advancing years. This is due to a loss of the positive 
refracting power of the eye, which diminishes rapidly after the 
age of fifty, the emmetrope becoming hypermetropic, the hy- 
permetrope more hypermetropic, and the myope becoming 
emmetropic, or, possibly, hypermetropic. The vision of this, 
eye, therefore, indicates emmetropia, but we will whisper in 
our friend’s ear that emmetropia at this age does not exclude 
the possibility of myopia at fifty years, but only that this eye 
seems to have normally undergone the usual changes accom¬ 
panying advancing years. In fact, it may be considered as 
altogether probable that a slight myopia (perhaps 1 D.) did 
exist, and that it was neutralized by the loss of refraction and 
passed over into emmetropia, as is usually the case. 

Our optician now examines the glasses the patient has 
been using, and finds them to be + 2.25 D., and he expresses 
his surprise that the patient ishould have been able to use these 
(rather weak) glasses for so long a time. He says he can read 
large letters with them for a short while if he has a very good 
light, but he cannot continue reading for any length of time. 
On being asked to show how he reads, it is noticed that he 
holds the book at a great distance from the eyes, and prefers to 
place the lamp between his eyes and the book. With his + 2.25 
D. glasses he is able to read at a distance of sixteen inches for 
a little while. 

Although the patient does not know the reason why, the 
lamp is placed in the position mentioned so that its strong 
light will fall upon the pupils and contract them, and in this 
way shut off the circles of diffusion, which would otherwise 
disturb vision. These diffusion circles are caused by the 


238 


METHOD OF EXAMINATION. 


letters not being accurately focused on the retina, because of 
the lack of refractive power in the eye itself, and because of the 
insufficiency of the spectacle lenses that are worn to assist the 
eye; and the more of these diffusion circles that can be ex¬ 
cluded, the clearer will be the vision. 

The patient is asked at what distance he would like to 
read, or at what distance he is accustomed to read. He replies 
that of necessity he has been compelled to read at arm’s length, 
but that he would like to read at a distance of about twelve 
inches, and he wants to be able to read for any length of time 
and without discomfort. To be able to read at twelve inches 
without fatigue requires a positive refracting power of 3 D. 
(as shown in the previous case above). 

A careful examination shows that it is only the left eye 
that is able to read at the distance mentioned, the right eye, on 
account of its diminished acuteness of vision, not being able to 
participate in the act of reading. An ophthalmoscopic exami¬ 
nation of this eye reveals an incipient cataract, which explains 
the defective sight and which precludes the possibility of any 
benefit from glasses. 


ANOTHER CASE. 

The next patient is one that at once impresses the optician 
as being a myope. He wears glasses which are easily seen to 
be concave; his head is thrown back in the air, and his eyes are 
large and prominent. He says he has two brothers and a 
sister who are near-sighted, and another sister who is not. He 
has never been able to see well at a distance, and he has some¬ 
times used his brother’s glasses. 

On being asked if his parents are near-sighted, he replies 
in the negative, and he refers especially to his father as having 
had good sight, since he was seventy years old before he was 
required to use glasses, and he was always able to read the 
finest print. This, to the optician’s mind, is sufficient evidence 
of the existence of myopia, as no person of that age and of any 
other refraction would be able to read without glasses. 

The glasses the patient is wearing are found to be — 6 
D., and he -says they were fitted by an excellent optician and 
are the best that could be found. He boasts of his eyes as 


METHOD OF EXAMINATION. 


239 


being strong, sayis he has no disease of the eyes, is able to see 
the smallest objects, and can read the finest print; but com¬ 
plains of neuralgic pains which shoot through his eyes and 
forehead, but which he is hardly prepared to believe arise 
from his glasses. 

A book of fine print is then handed to the patient and 
be is asked how far away he is able to read the print, which 
distance is found to be eight inches, and which the optician 
knows corresponds to a myopia of 5 D. 

On the distance-card he is unable to read even the largest 
letters. As the reading test indicates a myopia of 5 D., glasses 
of this strength are placed before his eyes, and he is able to 
read the No. 30 line, and his vision is -§-§•. A weaker concave 
lens is not nearly so good, and a stronger concave- lens does 
not enable any more letters to be read, although it seems to 
make vision a little better. We, therefore, conclude that the 
myopia is equal to 5 D., and the examination of the other eye 
yields the same result. 

The optician now concludes that the patient has worn 
glasses that are too strong by 1 D., which have undoubtedly 
fatigued his eyes on account of the effort of accommodation 
required to overcome them. As he grows older the accom¬ 
modation becomes weaker, and it becomes less and less able to 
overcome the too strong lenses, until finally the fatigue and 
discomfort become intolerable and the patient is forced to the 
conclusion that something is wrong. 

In any case where it is impossible to raise vision to f-J- by 
spherical lenses, there is always a suspicion of astigmatism, 
and o-ur optician now proceeds to examine for that defect 
The card of radiating lines is placed on the wall, and each eye 
is examined separately with the — 5 D. 

In the examination and correction of our illustrative case, 
the optician is led to suspect the existence of astigmatism, and 
proceeds to examine for that defect by directing the patient’s 
attention to the card of radiating lines hanging on the wall 
while the patient wears the — 5 D. lenses, which were previ¬ 
ously found to be strong enough to correct his myopia. After 
a little hesitation he says he sees distinctly only the line on the 
right side at io°, while all the other lines are more or less in- 


240 


METHOD OF EXAMINATION. 


distinct, especially the one perpendicular to this, which is ioo 0 „ 
This, therefore, indicates astigmatism and gives the direction 
of the twoi principal meridians, one at io° and the other at ioo°. 
The optician knows that the refraction of the latter meridian is. 
—5, because the lines at right angles to this were seen clearly 
by a spherical lens of 5 D. 

In order to determine the refraction of the other principal 
meridian, the optician places over the — 5 D. spherical a con¬ 
vex cylinder with its axis at ioo°. This at once blurs the 
vision, and he then tries concave cylinders with axes at same- 
meridian. This at once improves vision, and after a little trial 
he finds that — .75 cyl., axis ioo°, affords the most satisfac¬ 
tory vision, makes all the radiating lines appear equally clear,, 
and raises the acuteness of vision to 

The correcting lenses for this case appear to be — 5 
spherical combined with — .75 cylinder, but the optician will 
find on trying both eyes together with the cylinders and 
weaker sphericals that — 4 affords almost as good vision as — 
5; and as the rule in myopia is to prescribe the weakest lenses v 
he orders — 4 S. 3 — -75 cyl., axis ioo°. These glasses are- 
advised for distance, and perhaps would also answer very well 
for reading for some years yet; but as our optician wants to 
avoid all cause of strain of the accommodation, and place the 
eyes under the most favorable conditions for use, he gives a 
weaker glass for reading, and orders — 2 S. 3 — *75 cyl., axis; 
ioo°. “These glasses give the greatest satisfaction,” was the 
report when the patient returned some time after. 

In describing these illustrative cases, we have endeavored 
to make them so plain and simple that the beginner in optics- 
would readily be able to follow each step and understand the 
rationale of it; and no optician can carefully read them without 
gaining much practical information. We will conclude this- 
series of practical examples in the determination of refraction,, 
accommodation and visual acuteness with 

ONE MORE ILLUSTRATIVE CASE. 

This is a young man fifteen years of age. His sight has; 
always been poor both at a distance and close at hand, and! 


METHOD OF EXAMINATION 


241 


although he has tried to get glasses, he has never yet been able 
to find any which would improve his vision very much. He 
has just commenced his college course and taken up special 
studies in mathematics, and now the deficiency of sight is 
becoming a serious obstacle. He is unable to see the charts 
on the wall as the other students do, and he finds the greatest 
difficulty in making the correct geometrical drawings. On a 
general inspection,nothing abnormal is noticed about his eyes; 
they look perfectly natural, and any one would infer their pos¬ 
sessor enjoyed good sight. But our optician notices an asym¬ 
metry of the face which is quite evident to a careful observer. 

The reader who has carefully followed our description of 
the case so far will instinctively think of astigmatism as possi¬ 
ble cause of the trouble, for the following reasons: Imperfect 
vision both near and at a distance, inability to be suited with 
the ordinary spherical glasses that are kept in stock at the 
stores, difficulty in making exact mathematical drawings, and 
asymmetry of the features of the face, all of which are symp¬ 
toms of astigmatism. 

Following out this suspicion, our optician proceeds to ex¬ 
amine the eyes according to the rules already given. The 
right eye is able to read the letters on the No. 100 line with 
considerable difficulty, and if he attempts to name the letters 
on the next line he makes some amusing mistakes. He con¬ 
founds letters which have no resemblance to each other, and 
sometimes is able to distinguish a complicated letter while he 
is unable to make out a very simple one by its side. 

Convex glasses are first tried, in accordance with the 
proper routine of making an examination of the acuteness of 
vision, and it is found that + 1 D. improves the sight and 
raises the visual acuteness to f-jj-, which cannot be improved 
any further by convex glasses, as stronger ones begin to blur 
and dim the letters. 

With this + 1 D. lens before his eye the patient’s atten¬ 
tion is directed to the card of radiating lines hanging on the 
wall. He sees only the vertical line distinctly, and even that 
is not perfectly black; all the other lines are confused and ill- 
defined. Our optician now adds a + .50 cylinder with its axis 
in the direction of the line which is least distinct and at right 


242 


METHOD OF EXAMINATION. 


angles to the line which appears clearest. He knows that the 
vertical line is seen by the horizontal meridian of the eye, and 
the horizontal line by the vertical meridian; and when he 
places the cylinder with its axis in the direction of the line 
which is least distinct (that is, + .50 cyl., axis 180°) he adds to 
the refraction of the vertical meridian in the effort to improve 
the horizontal lines. This, however, makes vision worse and 
the lines appear to be more confused. The inference to be 
drawn from this is that the vertical meridian is not hyperme¬ 
tropic, because the horizontal line is not improved by the + 1 
spherical and is made still worse by the + .50 cyl., axis 180°. 

The optician now very properly rotates the cylinder be¬ 
fore the eye in the endeavor to find a position in which the 
lines will be improved and made to appear more nearly alike. 
When the cylinder is rotated at right angles to its present 
position (that is, at 90°), the vertical line is made still more dis¬ 
tinct than it was with the spherical alone. The inference to be 
drawn from this fact is, the horizontal meridian is still more 
hypermetropic than 1 D. The optician now tries stronger 
and stronger convex cylindrical glasses with their axes in the 
same meridian, with the result of making the vertical line 
still more distinct, the other lines also becoming clearer. Fi¬ 
nally he reaches + 4 D. cyl., axis 90°, which seems to be 
about the strongest convex cylinder the eye will bear, with 
which all the lines seem quite clear except the horizontal one. 

With this combination (+ 1 D. spherical combined with 
+ 4 D. cyl., axis 90°) the acuteness of vision is raised to f^-. 
This is certainly a very great improvement over that obtained 
by the spherical alone, and yet our optician must not be con¬ 
tent with that; in fact, he would not do his whole duty unless 
he made an effort to raise the acuteness of vision still higher, 
because he must know that the vertical meridian is not prop¬ 
erly corrected or the horizontal line would appear as clear as 
the vertical. 

Up to this point the vertical meridian of the cornea ap¬ 
pears to be hypermetropic 1 D., which is the strength of the 
spherical lens first placed before the eye; but the dimness of 
the horizontal line proves that this 1 D. is not the proper cor- 


METHOD OF EXAMINATION. 243 

rection for this meridian, and an effort must now be made to 
ascertain how it can be improved. 

Perhaps this meridian may be emmetropic, and perhaps 
all the defect is in the horizontal meridian. With the sphero- 
cylinder now before the eye (+ i D. sph. C + 4 D. cyl., axis 
90°) the horizontal meridian is corrected by + 5 D. and the 
vertical meridian by + 1 D. If our optician wishes to deter¬ 
mine if the vertical meridian is emmetropic, that is, if he 
wishes to correct only the horizontal meridian, he removes the 
above sphero-cylinder and replaces it with a plain cylinder, 
+ 5 D. cyl., axis 90°, with which the lines are seen pretty much 
as they were before, or perhaps a shade clearer. The inference 
to be drawn from this is that the vertical meridian is neither 
hypermetropic nor emmetropic, and the suspicion is very 
properly raised that it may be myopic. 

In order to determine whether this meridian is myopic, 
•concave cylinders are used and placed in the trial-frame with 
their axes horizontal or at 180°. A — .50 cyl., axis 180 0 , is 
added, and the vision is at once very markedly improved and 
the horizontal line made very much clearer. A — .75 cyl., 
axis 180 0 , produces still further improvement and makes the 
horizontal line appear as clear as the vertical. This makes a 
cross-cylinder, + 5 D. cyl., axis 90°, combined at right angles 
with a — .75 cylinder (+ 5 D. cyl., axis 90°, L — -75 cyl), 
which raises vision to 

This case, therefore, proves to be one of mixed astigma¬ 
tism, with a hypermetropia of 5 D. in the horizontal meridian 
and a myopia of .75 D. in the vertical meridian, and is cor¬ 
rected by the above formula. Mixed astigmatism may also 
be corrected by a sphero-cylinder, or, in other words, this 
cross-cylinder may be reduced to a sphero-cylinder as follows: 
— .75 D. S. C + 5.75 D. cyl., axis 90°. 

The impairment of the acuteness of vision by this degree 
of astigmatism is very great, while the restoration of vision by 
the proper combination of lenses is highly satisfactory, and is 
one of the most agreeable experiences with which the optician 
can meet. An original vision of y 2 ^ is raised to after cor¬ 
rection, and a new world is opened up to the hitherto un- 


244 


METHOD OF EXAMINATION. 


fortunate patient. Surely this is a wonderful achievement for 
scientific optics. 

It is a very interesting fact that this patient’s mixed astig¬ 
matism can be converted into a case of simple astigmatism bv 
calling his accommodation into action. In such a case the ex¬ 
ercise of the accommodation corrects the 5 D. of hyperme- 
tropia in the horizontal meridian, while at the same time it 
increases the myopia in the vertical meridian to the same de¬ 
gree (5 + .75 = 5.75), and, therefore, the case is now corrected 
by — 5.75 cyl., axis 180°. This affords a normal degree of 
vision ( V. = -f-g-), but it imposes a great tax on the accommo¬ 
dation, and would soon cause symptoms of asthenopia if the 
patient used such a glass. 

Another interesting fact about this case is that the patient 
can neutralize his astigmatism and raise his vision to the nor¬ 
mal standard by applying the tip of his finger on a point of the 
eye-ball upward and outward, the pressure exerted on this 
point being in the direction of one of the principal astigmatic 
meridians. 

The subject of mixed astigmatism and its correction by 
crossed cylindrical lenses has always been considered a diffi¬ 
cult and complicated one, and rightly so. The subject will be 
more fully considered in the chapter on astigmatism, but the 
preceding remarks will be of much practical value in pointing 
out one of the methods of the detection and correction of this 
defect. 


RECAPITULATION. 

This concludes the examination of distant vision, or, in 
other words, the testing of the acuteness of vision. It is a 
matter of the greatest importance to every person, especially 
to those whose occupation requires a continued use of the eyes 
(and what occupation or business does not require a constant 
use of these organs?), to be informed of the exact state of 
affairs with regard to his acuteness of vision. The educated 
optician must be competent to make the necessary examina¬ 
tions, and he stands prepared to furnish this information to 
any and all who desire to know the state of their vision and 
who seek his skill. 


METHOD OF EXAMINATION. 


245 


There is but one way in which this knowledge can be 
gained, and that is for the individual to apply to a skilled 
optician who is competent to subject the eyes to certain tests 
which he knows can be relied upon. If they can pass these 
tests successfully, it will be a source of no little satisfaction for 
the patient to know that all is right. 

If, on the other hand, the eyes are unable to measure up 
to the standard as required by the test, it indicates that some¬ 
thing is wrong. The degree of departure from normal vision 
is ascertained at the same time, and the patient is thus warned 
that his eyes need attention and assistance in order to avoid a 
possible failure of sight. 

The patient’s vision is either normal and up to the stand¬ 
ard, or it is abnormal and below the standard. 


Normal Vision, j 

or a vision of j 


means 


r Emmetropia 
■] or possibly 
v Hypermetropia. 


Subnormal Vision, 
or a vision 
below 


Hypermetropia. 

Myopia. 

Astigmatism. 

Amblyopia. 

Spasm of Accommodation. 
Opacities of some of the 
Media. 

Organic Disease. 


TESTING THE ACCOMMODATION. 

After having ascertained the acuteness of vision and de¬ 
termined the refraction of the eye under examination, the 
optician naturally passes on to estimate the accommodation 
and to test the eye for vision close at hand as compared with 
that at a distance. 


246 METHOD OF EXAMINATION. 



Relative difference in the size of the retinal images of 1, Emmetropla; 

2, Corrected Hypermetropia; 3, Myopia of 4 D. 

For this purpose the near type is used, or reading matter 
with types of different sizes, with which to ascertain and de¬ 
termine the near point and far point at which any particular 
line or size of type can be read. 

Some authorities object to the employment, for this pur¬ 
pose, of bits of reading matter, for the reason that reading is 
not a certain proof of visual acuteness. They say, and with 
much reason, that persons who are accustomed to reading are 
able to guess at the majority of the words by their general 
aspect and by their relations to neighboring words, while 
those who are but little educated and unaccustomed to reading 
must decipher the letters one by one, and are, therefore, rela¬ 
tively in more unfavorable conditions while undergoing this 
examination than the former. 

They say, further, that if we wish to determine the acute¬ 
ness of vision at short distances, we should use isolated letters 
constructed on the same principles as the larger test-types. It 
is also evident that we must make the examinations always at 
the same distance, in order to obtain results exact and compar¬ 
able. This examination would then be based on the same 
principle as that at a distance, namely, on the equality in the 
size of the retinal image. 

A second point, and a most important matter, is that the 
vision near at hand is very different in the different states of 
refraction. We will take, for example, a distance of ten 
inches. The young emmetrope will see at this distance with 
the aid of his accommodation, while the hypermetrope and the 
presbyope will also be able to see with the aid of convex 
glasses of greater or less strength, according to the power of 
their accommodation and the degree of their ametropia. The 


METHOD OF EXAMINATION. 


247 


myope whose far point is at a distance greater than ten inches 
will, likewise, have need of a slight effort of accommodation. 
Only a myope of 4 D. will be able to see at a distance of ten 
inches without any effort of accommodation and without any 
correcting glasses. Degrees of myopia greater than 4 D. will 
require the use of concave glasses for seeing at the same dis¬ 
tance. 

It is obvious, therefore, that vision under these circum¬ 
stances is attended with notable differences in the size of the 
retinal images. It makes no difference if the same test-letters 
are employed and if they are placed at exactly the same dis¬ 
tance; the emmetrope, who accommodates, will still have ret¬ 
inal images smaller than the presbyope or the hypermetrope, 
who uses glasses, and the hypermetrope smaller images than 
the myope of 4 D. The result is, therefore, vitiated, as the 
size of the retinal image is changed in each case. 

There is a method (by testing the eyes at a fixed distance 
with a convex lens of the same focal distance) by which it is 
possible to obtain in near vision the same advantages as in 
distant vision; that is, equality in the size of the retinal images, 
exclusion of the accommodation, and simultaneous determina¬ 
tion of the refraction and of the accommodation and visual 
acuteness. But it is a very difficult matter to exclude the ac¬ 
commodation in near vision, even by the aid of a strong 
convex lens, except by the employment of atropine; and, con¬ 
sequently, this method of testing the vision is not generally 
employed in practice, and, therefore, we will not devote any 
space to a description of it. 

In contrast to this method we have the reading types, 
which are in extended use and which serve every practical 
purpose, because all the optician desires is to be able to make 
such an examination as will enable him to prescribe those 
glasses which will allow the patient to use his eyes and do his 
work with comfort. 


AMPLITUDE OF ACCOMMODATION, 

The nearest point at which the reading matter can be dis¬ 
tinguished, that is, the closest point for which the eye can 


248 


METHOD OF EXAMINATION. 


accommodate itself, is called the near point. When the eye is 
in a condition of perfect repose and its accommodation en¬ 
tirely relaxed, it is then adjusted for the greatest distance at 
which it is able to see, which is called the far point, which, in 
emmetropia, is said to be at infinity. 

(The hypermetropic eye is adjusted for a point beyond 
infinity, which means it is adjusted for rays converging toward 
its far point. The myopic eye has its far point at a certain 
fixed distance in front of it, and its dioptric system is adjusted 
for that distance.) 

For practical purposes and in every-day examinations'of 
the accommodation of the eye, the optician can call that point 
the far point which is at the greatest distance at which the 
reading type can be distinguished. 

The distance between the near point and the far point is 
called the range of accommodation. It is the distance over 
which the eye has command by the aid of its accommodation. 
The force necessary to change the eye from its far point to its 
near point is called the amplitude of accommodation. There¬ 
fore, the amplitude of accommodation is represented by the 
difference between the refraction of the eye when in a state of 
complete rest and when at its maximum of accommodation. 

The action of the accommodation in focusing the eye for 
its near point is of the same effect as a convex lens which 
would enable an eye, deprived of its accommodation, to see at 
the same point, and, therefore, the strength of the accommo¬ 
dation can be expressed by the number of this lens. In other 
words, it may be said that the accommodation is equal to a 
convex lens of .such a strength as would give to rays coming 
from the near point a direction as if they came from the far 
point. 

In view of the fact that the accommodation is equal to a 
convex lens of such power as to give rays coming from the 
near point a direction as if they came from the far point, the 
question naturally occurs, what will be the strength of such a 
lens? 


METHOD OF EXAMINATION. 


249 



diagram of an emmetropic eye, showing its adaptation for parallel rays, 
and the action of a convex lens in making divergent rays parallel. 


An emmetropic eye, with its far point at infinity, is 
adapted for parallel rays proceeding - from D, as shown in 
above diagram. In this condition of the eye no other rays but 
parallel can be focused on the retina. Rays proceeding from 
any other point, as N , must be rendered parallel before they 
can be united at the proper point on the retina. 

Now every student of optics knows that parallel rays 
passing through a convex lens are brought to a point at the 
principal focus of the lens; and conversely, the divergent rays 
proceeding from the principal focus of the lens will emerge 
from it parallel. Therefore, if we place in front of this emme¬ 
tropic eye a convex lens whose focus would be at N, it would 
render the rays proceeding from N parallel, or just as if they 
came from D. The eye, therefore, by the aid of this convex 
lens will see as well at the near distance N, as it does without 
the lens at the far distance D. 

From the above facts the following rule is deduced: For 
the emmetropic eye the focus of the lens coincides with the 
near point, in order that it may render parallel the divergent 
rays proceeding from that point. Its focal distance is there¬ 
fore equal to the distance which separates the near point from 
the eye. If this distance is twenty-five centimeters, the lens 
will have a refracting power of = 4 D., and the amplitude 
of accommodation will be equal to 4 D.; or, if the distance is 
measured in inches, it will be found to be at ten inches (which 
is equivalent to twenty-five centimeters), which means a re¬ 
fracting lens of one-tenth inch (which is equivalent to 4 D.). 

The exercise of this refracting power serves to adapt the 
eye for points situated nearer than infinity. In order to deter¬ 
mine the amplitude of accommodation of an emmetropic eye, 









250 


METHOD OF EXAMINATION. 


we have only to find the nearest point at which the patient is 
able to read the smallest sized type. The distance of this point 
is the focal distance of the lens corresponding to (and express¬ 
ing) the amplitude of accommodation. 

This may be expressed in inches if the optician is accus¬ 
tomed to use the inch system of numbering lenses. If, then, 
the distance of this point be ten inches, the amplitude of ac¬ 
commodation will be equal to a convex lens of T y inch focus. 
If at eight inches, to a convex lens of -J- inch focus, and so on. 

If, on the other hand, the optician is familiar with the 
dioptric system of numbering lenses, he will measure the dis¬ 
tance of this near point in centimeters, as marked on his 
metric rule. The division of one hundred by this figure will 
give the strength of the lens, numbered in dioptres, which in 
an emmetropic eye expresses the amplitude of accommodation 
(and also its positive refracting power). For instance, if the 
near point be found at 20 centimeters, the amplitude of accom¬ 
modation will be 5 D. (-W“ = 5 D.). 

MEASURING ACCOMMODATION BY CONCAVE LENSES. 

The strength of the accommodation can also be measured 
by means of a concave lens. Now it is a well-known fact to 
every optical student that a concave lens causes parallel rays of 
light to diverge as if they proceeded from a point near at 
hand; and the stronger the lens the more the divergence, be¬ 
cause the rays have the same direction as if proceeding from 
the focus of the lens. In order, therefore, to preserve distinct 
vision through such concave lens, an eye must use the same 
accommodative power as it does when looking at an object 
situated at the focus of this concave lens. 

As an illustration, a concave lens of 4 D. will give to par¬ 
allel rays of light such a direction as if they diverged from a 
point situated ten inches back of the lens; and therefore an eye 
will have to bring into play the same amount of accommo¬ 
dative power in looking through such a lens as would be re¬ 
quired in looking at an object situated at a distance of ten 
inches, the reason being that in each case the rays of light enter 
the eye of the observer under exactly the same degree of di- 


METHOD OF EXAMINATION. 


251 


vergence. The action of the accommodation must increase 
the refracting power of the eye sufficiently to overcome or 
neutralize the effect of the concave lens precisely in the same 
degree as this concave lens tends to diminish the positive re¬ 
fracting power of this eye. For this reason an emmetrope 
who looks at distant objects through a concave lens experi¬ 
ences the same sense of fatigue in his eyes as when looking at 
an object close at hand. 

Therefore the strongest concave lens through which an 
emmetropic eye is still able to see clearly at a distance is the 
measure of the amplitude of its accommodation. An emme¬ 
tropic eye which is able to overcome an 11 D. concave lens in 
looking at a distance, possesses an amplitude of accommoda¬ 
tion of 11 D. and its near point is situated at 9 centimeters in 
front of the eye ( -y-y 0 - — 9)> f° r the reason that this concave lens 
of 11 D. causes parallel rays to diverge as if they came from 
its focus, which is 9 centimeters behind it. 

If an emmetropic eye is able to overcome a concave lens 
of i inch focus, it will have an amplitude of accommodation 
equal in strength to a convex lens of the same focal distance, 
and its near point will be situated at five inches in front of the 
eye, since this particular concave lens causes parallel rays to 
diverge as if they proceeded from its focal point, which is five 
inches behind it. 


THE ACCOMMODATION IN HYPERMETROPIA. 

The hypermetropic eye in a condition of repose is lacking 
in refractive power; in other words, presents a deficiency of 
refraction. In order for such an eye to be able to see at a dis¬ 
tance, or in order to make such an eye emmetropic, either one 
of two conditions must be present: either a convex lens must 
be supplied equal in strength to the deficiency of refraction, or 
an equal effort of accommodation must be made. 

A hypermetrope, therefore, who wishes to see at the same 
distance as an emmetrope, must accordingly employ a part 
more of his accommodation than the emmetrope (sufficiently 
more to neutralize or correct the deficiency of refraction). 
Consequently, in expressing the amplitude of accommodation 


252 


METHOD OF EXAMINATION. 


of a hypermetropic eye, the power necessary to adjust such an 
eye for infinity must evidently be added to that which changes 
its adjustment from infinity to the near point. 

As a practical illustration, we will consider the amplitude 
of accommodation of a hypermetrope of 2 D., whose near 
point is situated at 20 centimeters. Such an individual must 
use 2 D. of accommodation to adjust his eyes for distance or 
to render them emmetropic. Now to adjust an emmetropic 
eye for a near point of 20 centimeters, there is required 5 D. of 
accommodation ( W 0 - = 5 D.). Therefore, in this particular 
case the amplitude of accommodation would be 5 D. + 2 D. = 
7 D. 

Or, perhaps, it can be made plainer to some readers by 
using the inch numbers, in which case we have a hyperme¬ 
trope of 2-V, whose near point is situated at eight inches. He 
is required to use his accommodation to the extent of a + yV 
lens for distance, and to the extent of a + -J lens for the near 
point of eight inches, and his amplitude of accommodation is 
2V + i = to > which means a convex lens a little stronger 
than + -J. Calculations such as these emphasize the difficulty 
of working with the vulgar fractions required by the inch 
system, and serve to point out one of the many advantages of 
the metric system. 

What is the near point of a hypermetrope of 3 D., who 
possesses an amplitude of accommodation of 6 D.? Inasmuch 
as this patient is required to use 3 D. of accommodation to see 
at a distance, there remain only 3 D. to adjust the eye for near 
vision. His near point, therefore, would be 33 centimeters 

( 4 * = 33 ). 

Using the inch system, the question would be, What is 
the near point of a hypermetrope of T V inch who possessed an 
amplitude of accommodation of J inch? After using T V to 
overcome his hypermetropia there would remain y 1 ^ for near 
vision (jt — y?jr — tV)> which would place his-near point at 
twelve inches. 

From these illustrations it can be seen that a hyperme¬ 
trope of 3 D., or of y 1 ^, although respectively possessing an 
amplitude of accommodation of 6 D. and of J inch, is able to 
see no nearer than an emmetrope possessing an amplitude of 


METHOD OF EXAMINATION. 


253 


only 3 D. or X V inch. And reasoning from the same stand¬ 
point, if an emmetrope and a hypermetrope are able to see at 
the same near point, the hypermetrope must possess the 
greater amplitude of accommodation. If a hypermetrope of 
2 D. and an emmetrope both have their near points at 20 cen¬ 
timeters (eight inches), the former will have 2 D. more of 
accommodation. In order to see at that distance the emme¬ 
trope will require 5 D. of accommodation (-^°- = 5 D.), while 
the hypermetrope will have 5 + 2 = 7 D. This shows the dis¬ 
advantage under which a hypermetropic eye constantly labors 
and its need of a surplus of accommodative powers. 

THE ACCOMMODATION IN MYOPIA. 

In looking at the same near point, the myopic eye will re¬ 
quire less accommodation than the emmetropic eye, because, 
when at rest, the latter is adjusted for distance, and the former 
for some definite point close at hand; and, therefore, at this 
point the myopic eye will see without any effort of accommo¬ 
dation, while the emmetropic eye will have to call into use part 
of its accommodation. 

Therefore, in order to determine the amplitude of accom¬ 
modation of a myopic eye, it will be necessary to subtract the 
refracting power by which the myope surpasses the etnme- 
trope from that which would be required to adjust the emme¬ 
tropic eye to the near point of the myope. In other words, 
the amplitude of accommodation which is normally present in 
an emmetropic eye will, in a myopic eye, be diminished by the 
amount of myopia present. 

As an illustration, a case of myopia of 10 D. may be taken 
with a near point of 7 centimeters. Now, a near point of 7 
centimeters in an emmetrope calls for an amplitude of accom¬ 
modation of 14 D. ( Ap = 14 D.). In this case of myopia, 10 
of the 14 D. of accommodation required are supplied by the 
error of refraction, and, consequently, the amplitude of accom¬ 
modation is the difference between the two, 14 D. 10 D. 
4 D. 

Or a case of myopia of one-tenth inch may be taken, with 
a near point of six inches. In emmetropia a near point of six 
inches represents an amplitude of accommodation equal to a 


2 54 


METHOD OF EXAMINATION. 


convex lens of one-sixth inch, and, as part of this required 
convexity is supplied by the myopia, the amplitude of accom¬ 
modation would be the difference between the two, ■£■ — T V = 
T5 i nc h* 

“It’s a poor rule that won’t work both ways,” and there¬ 
fore the total amount of refractive power possessed by a my¬ 
opic eye is the sum of its amplitude of accommodation and its 
myopia. For instance, a myope of 4 D. with an amplitude of 
accommodation of 6 D. possesses a positive refracting power of 
10D. (6 + 4= 10D.), which represents a near point of 10 cen¬ 
timeters (-VV- = I0 )> or four inches. 

CONVERGENCE. 

Having now completed his test of the accommodation, 
and having studied it not only as it exists in emmetropia, but 
also as it is affected by myopia and hypermetropia, the opti¬ 
cian must remember that this is not the only factor or function 
which is called into play in binocular near vision, and his at¬ 
tention must be directed to the function of convergence, inas¬ 
much as these two functions go hand in hand in the use of the 
eyes on any object nearer than infinity. 

The function of accommodation is brought into action to 
adjust the refractive condition of the eye for vision at close 
distances, and the function of convergence is then called into 
play to alter the direction of the two eyes and place them in 
such a position that the image of the object looked at may fall 
on the yellow spot of each eye. 

These two functions, accommodation and convergence, 
bear a constant relation to each other, within the limits of the 
amplitude of accommodation on the one hand and the ampli¬ 
tude of convergence on the other. As an object approaches 
the eyes, the accommodation and convergence are instinct¬ 
ively called into action; and the closer the object, the stronger 
must be the effort of both these functions. As vision is again 
turned to distant objects, the two functions relax in equal 
proportion. 

For every increase of accommodation there is a corre¬ 
sponding increase of convergence, and the simultaneous ac¬ 
tions of the muscle of accommodation and of the internal recti 


METHOD OF EXAMINATION. 


2 55 


muscles are so intimately associated the one with the other, 
that neither function can be satisfactorily used separately and 
independently. It would be an extremely difficult matter to 
converge without accommodating or to accommodate without 
converging. 


D I 771 D 



THE METER ANGLE. 

In the above illustration, when the two eyes are directed 
to distant objects (marked D), the visual lines will be parallel 
and there will be no angle of convergence. 

When the eyes are directed to an object one meter away 
and the visual lines of the two eyes made to converge to this 
point, a certain angle of convergence will be formed by the 
meeting of the visual line of each eye with the median line, 
which is called The Meter Angle. It expresses the degree of 
convergence which is required to maintain binocular vision at 
that distance, and may be employed as the unit from which to 
express other degrees of convergence. In this case the 
metrical angle equals one, and is written as follows: C. = i 
(the C. being the sign for convergence). 






256 


METHOD OF EXAMINATION. 


If the object looked at be situated half a meter from the 
eye, the angle of convergence must, of necessity, be twice as 
large as when at one meter, and then we have C. = 2. If the 
object be brought closer, so as to be placed at a distance of 
one-third of a meter from the eye, the angle of convergence 
must be increased in the same proportion, and then we have C. 
— 3. If, on the other hand, the object of attention be situated 
at a greater distance than one meter, say at two meters or at 
four meters, the angle of convergence would be proportion¬ 
ately diminished and then we would have C. = -J, or C. = 
And just in proportion as the object is situated at a greater 
distance from the eyes will the angle of convergence diminish, 
until finally, when infinity has been reached, the angle of con¬ 
vergence will have disappeared and the visual lines become 
parallel. 

When the eyes are directed to the closest point at which 
they are able to see distinctly, that is, the nearest point of bin¬ 
ocular vision, the angle of convergence is at its greatest, and 
may be said to be adapted for its near point. When the eyes- 
are directed to infinity, the angle of convergence is at its least; 
in fact, there is no longer any angle at all. The distance be¬ 
tween these two points represents the range of convergence. 
The far point of convergence is always situated at infinity, or 
even beyond. * The amplitude of convergence is the whole 
amount of convergence that can be exerted by the strongest 
effort of the internal recti muscles. 

Now it is an established fact that the average normal em¬ 
metropic eye requires, for each point of distance of fixation of 
binocular vision, as many meter angles of convergence as it 
requires dioptres of accommodation; this refers to each and 
every point of fixation nearer than twenty feet. For a distance 
of one meter there is required an exercise of 1 D. of accommo¬ 
dation in emmetropia in order to focus the image on the retina; 
and at the same distance there will be required one meter 
angle of convergence in order that the internal recti muscles 
may so converge the eyes that their visual axes may cross at 
this point and thus form the image on the yellow spot of each- 
eye. If the object be situated at a distance of half a meter, 
there will be required an exercise of 2 D. of accommodation* 


METHOD OF EXAMINATION. 


257 

which corresponds to two meter angles of convergence. If at 
one-third a meter, we have 3 D. of accommodation and 
C. = 3. 

The following table, taken from “Hartridge on Refrac¬ 
tion,” shows the angle of convergence in degrees for the dif¬ 
ferent distances of an object when the eyes are 6.4 centimeters 
apart, which corresponds to a pupillary distance of about two 
and a half inches. 


Distance of the object 

The Metrical 

Value expressed 

from the eyes. 

Angle. 

in degrees. 

i Meter 

I 

I° 50 ' 

50 Cm. 

2 

3 ° 40' 

33 “ 

3 

5 ° 30 ' 

25 “ 

4 

7° 20' 

20 “ 

5 

9° 10' 

16 “ 

6 

ii° 

14 “ 

7 

12 0 50' 

12 • “ 

8 

14 0 40' 

11 “ 

9 

16 0 30' 

10 “ 

10 

18 0 20' 

9 “ 

11 

20° 10' 

8 “ 

12 

22° 

7.5 " 

13 

23 ° 50 ' 

7 “ 

14 

25 ° 40 ' 

6-5 “ 

15 

27 ° 30 ' 

6 “ 

16 

29 0 20' 

5-5 “ 

18 

33 ° 

5 “ 

20 

36 ° 40' 


Although the functions of accommodation and conver¬ 
gence are so intimately connected and are exercised so com¬ 
pletely in unison, yet this relation is not absolutely fixed and 
invariable, but it is possible for either function to be brought 
into use and exercised within certain limits independently of 
the other. 

THE RELATION BETWEEN ACCOMMODATION AND CONVER¬ 
GENCE. 

The functions of accommodation and convergence, al¬ 
though so intimately connected, may within certain limits be 
used independently of each other. For instance, the accom¬ 
modative effort may be increased or diminished, while the 


258 


METHOD OF EXAMINATION. 


object is kept distinctly in view and the same degree of con¬ 
vergence maintained. This fact can be proven by the use of 
convex and concave lenses. 

When a concave lens is placed before the eye, the accom¬ 
modation is at once called into action to neutralize or over¬ 
come the diminishing effect of the negative lens and to enable 
the object to be still distinctly seen. This increase of accom¬ 
modation takes place without a corresponding increase of con¬ 
vergence. 

When a convex lens is placed before the eye, the accom¬ 
modation relaxes as much as possible, because the positive 
lens supplies all the refractive power needed without any effort 
of the accommodation, and if the lens is not too strong the 
object can still be clearly seen. This relaxation of accommo¬ 
dation takes place without a corresponding diminution of con¬ 
vergence. 

And, on the other hand, the convergence may be altered; 
it may be either increased or diminished, without any corre¬ 
sponding change in the accommodation. This fact can be 
proven by trial with a prism base in and a prism base out. 

When a weak prism is placed before the eye with its base 
inward, it bends the rays of light toward its base, and in doing 
so it relieves the convergence to that extent; and, therefore, 
in order that double vision may not be produced, it becomes 
necessary for the eye before which the prism is placed to rotate 
outward. Now it has been ascertained that an individual is 
able to do this without destroying the distinctness with which 
the object is perceived. This proves that the angle of conver¬ 
gence has been lessened without a corresponding diminution 
of accommodation. 

When a weak prism is placed before the eye with its base 
outward, the rays of light will again be bent toward its base, 
which in this case is outward. This calls for an extra effort of 
convergence in order to overcome the outward tendency of 
the prism and to maintain binocular vision, and therefore the 
eye-ball must, of necessity, be rotated inward by the in¬ 
creased action of the internal recti muscles. It has been found 
possible for this to be done without interfering with the clear¬ 
ness of vision, which means without any change in the accom- 


METHOD OF EXAMINATION. 


259 


modation, a fact which goes to prove that the angle of con¬ 
vergence can be increased without a corresponding increase of 
accommodation. 

POWER OF CONVERGENCE. 

In early life the normal eyes possess a power of conver¬ 
gence of eighteen or twenty meter angles or more; but, of 
course, there are variations in this which are dependent some¬ 
what on the condition of the refraction. For instance, a 
hypermetropic person, whose eye-balls are necessarily shorter 
and the internal recti muscles usually well developed, is able 
to exercise a greater amount of convergence in proportion 
than a person with emmetropic eyes; while, on the other hand, 
a myopic individual, whose eye-balls are longer and whose 
internal recti muscles are apt to be weak, is unable to exercise 
as great an amount of convergence as a pair of emmetropic 
eyes. For the convenience of opticians tape measures have 
been prepared, which are marked on one side with centimeters 
(or inches) to show the distance at which an object is held and 
the point to which the eyes must converge, while the opposite 
side is marked with the corresponding meter angle. 

For the convenience and comfort of reading and near 
work, a certain amount of positive power of convergence is 
required, which should be double that required for the point 
or distance at which the reading or work is accustomed to be 
held. By way of illustration, if a patient reads or works at 
one-third of a meter from his eyes, that is, at a distance of 
thirteen inches, he will require three meter angles of conver¬ 
gence for that distance; or, in other words, C. = 3. 

But if this was all the positive converging power pos¬ 
sessed by this individual, he could not, for any length of time, 
or with any degree of comfort, keep his eyes converged to this 
point. No one is able to keep in use the full power of his 
convergence, any more than he is able to keep up the use of 
his full power of accommodation, and neither of these is 
possible. There must be a certain amount of power of con¬ 
vergence held in reserve, just as there must be a certain re¬ 
serve amount of power of accommodation, and this should be 
twice as great as the power of convergence or accommodation 
employed. 


26 o 


METHOD OF EXAMINATION. 


In the above case, where three meter angles of conver¬ 
gence are required, the patient should possess at least three 
meter angles more of the power of accommodation in reserve 
in order to work or read comfortably at thirteen inches. 


DETECTION OF WEAK MUSCLES. 

In the study of the function of convergence and its de¬ 
parture from the normal condition, it is necessary to know 
how to determine the strength of the internal recti muscles, as 
well as to detect any weakness of these or of the other ocular 
muscles. 

The simplest method of testing any imperfection in the 
action of the ocular muscles is performed as follows: The 
head of the patient is to be held erect and he is asked to follow,, 
with his eyes, the point of a pencil or any other small object 
as it passes from right to left and from above downward. 

As the patient thus follows the pencil point, the observer 
must note not only whether each eye has a natural range of 
mobility, but also whether the two eyes move together in all 
directions. In attempting to do this the observer should re¬ 
member that the movements of the eyes in a normal condi¬ 
tion are something as follows: 

Outward, 45 to 50 degrees. 

Inward, 45 degrees. 

Upward, 35 to 40 degrees. 

Downward, 60 degrees. 

While there are various appliances for detecting and de¬ 
termining any limitation of the motion in one or both eyes in 
any certain direction, the observer is usually able to detect this 
by noticing whether or not the outer edge of the cornea 
reaches the canthus on the outer side (for example, when the 
eye is turned outward as much as its fellow), or whether the 
inner margin of the cornea of the other eye reaches the inner 
canthus; or, in other words, whether the movements of both 
eyes are exactly the same when regarded in connection with 
some definite fixed point. 


METHOD OF EXAMINATION. 26 l 

It must also be noted in the same connection whether or 
not the two eyes converge equally. For this purpose it is 
usually sufficient to hold the point of a pencil a few feet in 
front of the patient exactly in the median line, and then 
gradually approach it nearer to the patient while he is asked to 
look at it intently. As he does this he must, of necessity, use 
his internal recti muscles and direct both eyes inward; and if 
this act of convergence is performed in a perfectly normal 
manner, there should be no difference in the movements of the 
two eyes. They should both converge to a certain point, the 
power of convergence varying somewhat with the age and 
muscular strength of the patient, and then, when the object is 
approached so close to the eyes that single vision is no longer 
possible, both eyes should turn outward at the same time, 
while usually the patient complains of a feeling of discomfort 
and fatigue. But if any insufficiency should exist, then when 
the object is held at a distance of three or four inches from the 
eyes in the median line for a short time, that internal rectus 
muscle which is weaker than normal is unable to perform its 
function and relaxes its efforts, and the eye turns outward. 

THE TEST WITH PRISMS. 

The next test consists in measuring the ability of the eyes 
to overcome prisms. When a person with good eyes looks at 
an object at a distance the eyes are at rest, no accommodation 
and no convergence is required, and the eye-balls are directed 
straight ahead. If, now, a prism is held before one of the 
eyes, the rays of light passing through the prism will be bent 
toward the base of the prism, and the eye over which the 
prism is held will have to turn in the opposite direction in 
order to meet the entering ray, that it may be focused on the 
yellow spot of this eye and to correspond with the image 
formed in the other eye. 

If the prism (say of five degrees) be placed before the 
right eye with its base in, for example, then the rays of light 
passing through this prism will be bent inward, and it becomes 
necessary for the patient to rotate his eye slightly outward in 
order to meet the entering rays and bring them to a focus at 


262 


METHOD OF EXAMINATION. 



Showing the action of a prism base in, which relieves convergence and taxes 
divergence, and measures the strength of the external recti muscles. 

the yellow spot, which act is accomplished by the action of the 
external rectus muscle of this eye. 

If, on the other hand, the prism be placed before this eye 
with its base out, similar results are produced, only in this case 



Showing the action of a prism base out, which relieves divergence and taxes 
convergence, and measures the strength of the internal recti muscles. 





METHOD OF EXAMINATION. 


263 


it is the opposite muscle that is affected. Now the rays of 
light from an object are bent outward in passing through the 
prism, and it becomes necessary for the patient to rotate his 
eye slightly inward in order to preserve binocular vision, in 
which act he contracts the internal rectus muscle of this eye 
more than usual. 

These two conditions are well illustrated in the drawings, 
a careful study of which will be of much assistance to the opti¬ 
cian in understanding the action of prisms and our means of 
measuring the convergence and divergence of the eyes. 

METHOD OF MEASURING DIVERGENCE. 

This may be considered as a negative quantity (negative 
convergence) or as the minimum of convergence of the visual 
axes, which is really identical with their maximum of diver¬ 
gence. It also may be said to express the strength of the 
external recti muscles. 

The measure of the negative convergence or of the posi¬ 
tive divergence is determined by finding the strongest pair of 
prisms with their bases inward, which are compatible with 
single vision of a distant object. The deviating angle of each 
prism expresses the “absolute minimum of convergence” for 
each eye. 

The drawing illustrates how a prism with its base in causes 
the eye to rotate outward, thus making it an effort of the ex¬ 
ternal rectus muscle. If, now, the prism which is held before 
the eye be a very strong one, so strong that it is impossible for 
the external rectus muscle to draw the eye out far enough to 
meet the entering ray, then this ray will not fall upon the 
yellow spot, but will strike the retina on the inside of it, and 
thus double vision will be produced, one image being in its 
natural position, while the image of the right eye, being pro¬ 
jected outward from its wrong position, will be seen very 
much to the right. 

We have now described the action of a weak prism and of 
a strong prism. In the first case the eye rotates outward and 
binocular vision is preserved; in the second case the prism is 
so strong that the external rectus muscle is not powerful 


264 


METHOD OF EXAMINATION. 


enough to prevent diplopia. Between these two extremes, 
and after a careful trial, a prism can be found (perhaps 8° to 
io°) which is the strongest that will not produce double vision; 
the number of this prism will express the strength of the ex¬ 
ternal recti muscles and the ability of divergence. 

In measuring the divergent power of the external recti 
muscles, it is important that the point of fixation during the 
experiment should be at some considerable distance, if it is 
desired to ascertain the absolute minimum of convergence. 
The reason for this is that the knowledge on the patient’s part 
that the object is at a distance, removes from his mind any 
suggestion of the need of convergence, and to that extent 
assists the action of the external recti muscles, or allows them 
unhindered to exert their full divergent power. And an addi¬ 
tional reason is found in the close relation existing between 
accommodation and convergence, and the remoteness of the 
object relieving the convergence in the same proportion as 
the accommodation. 

If the patient under examination wears glasses for any 
existing optical defect, and if the glasses are not too weak, the 
experiment may be made of decentering them to gain their 
prismatic effect, and measuring the strength of the external 
recti muscles in this way. The base of the prism must be in, 
and, therefore, if the patient was myopic and wore concave 
lenses, they would have to be separated in order to secure the 
base of the prism in the proper direction. While if the patient 
was hypermetropic and wore convex lenses of sufficient 
strength, they would have to be approximated or decentered 
in for the same reason. 

As the patient looks through his glasses, the convex 
lenses are approximated or the concave lenses are separated, 
until double vision results. Now the optician has the strength 
of the glass and the distance it is decentered, from which he 
can find the degree of prism involved. An elaborate table has 
been prepared, showing the prismatic effect of decentered 
lenses, in which for every strength of lens from 0.50 D. to 20 
D., and for every millimeter of decentering from 1 millimeter 
to 32 millimeters, there is a corresponding and fixed equivalent 
in prismatic effect. 


METHOD OF EXAMINATION. 


265 


As an example, a patient may be taken who is wearing 
4 D. glasses, and these glasses may be separated until the 
distance between the optical centers of the lenses is 10 milli¬ 
meters more than the distance between the centers of the 
pupils of the two eyes. In this case each lens is displaced 5 
millimeters, and a reference to the table shows that a lens of 
4 D. with a decentering of 5 millimeters possesses a prismatic 
equivalent of i° 10'. This is expressed by 'saying that the 
maximum of divergence or the minimum of convergence is 
— i° 50'. 

It has been found that with vision at twenty feet or more, 
that is, when accommodation and convergence are normally at 
rest, the average power of the external recti (or abducting) 
muscles is measured by a prism of about 3°, sometimes a little 
less (6°) and sometimes a little more (9 0 ). This refers to em- 
metropia; or, if any optical defect is present, it must first be 
corrected by the proper lenses before the test is made. 

METHOD OF MEASURING CONVERGENCE. 

This has reference to positive convergence or the absolute 
maximum of convergence, and it may also be said to express 
the strength of the internal recti muscles, and it is determined 
by finding the strongest prism over one eye or the strongest 
pair of prisms over both eyes with their bases outward, which 
will not destroy single vision of a test-type or printed page 
held as close to the eyes as accommodation will permit. The 
optician should note that in this case the test is made with the 
object looked at as close as possible, in order that the accom¬ 
modation required to focus an object so close at hand will 
assist the convergence to reach its maximum. 

Another test, and an old established one it is, is for the 
optician to approach his finger toward the root of the patient’s 
nose, who keeps his eyes fixed upon the finger until it is so 
close that the maximum of convergence is reached, and then 
one eye will deviate outward. This test, however, scarcely 
discovers the absolute maximum of convergence, because as 
the object approaches the eyes diffusion circles begin to appear 
on the retina and increase in size as the object gets nearer, 


266 


METHOD OF EXAMINATION. 


Until the ciliary muscle gives up the impossible task of accom¬ 
modation, which then relaxes, and with it the convergence. 
The same thing occurs in absolute hypermetropia, where the 
efforts of accommodation cease as soon as the patient finds he 
is unable to exercise his accommodation to a sufficient degree 
to afford clear and distinct vision. 

In reference to the test first mentioned as the one on 
which to rely to measure the strength of convergence, some 
authorities call attention to> the chromatic dispersion pro¬ 
duced by the prisms as spoiling the clearness of the pictures 
produced on the retina, and therefore vitiating to some extent 
the value of the test. This can be avoided only by the use of 
monochromatic light, which is scarcely available by the opti¬ 
cian in his every-day work. 

RANGE OF CONVERGENCE. 

When the results of the tests for divergence and conver¬ 
gence, that is, the minimum of convergence and the maximum 
of convergence, are added together, the absolute range or 
amplitude of convergence is obtained, of which perhaps not 
more than one-third or one-fourth can be used continuously 
for comfortable vision in the ordinary occupations of life. 

The relative range of accommodation is determined by the 
strongest pair of prisms bases in and by the strongest pair of 
prisms bases out, which will not produce double vision of an 
object at some fixed distance. The distance most suitable 
would be that at which the patient’s daily work is done (occu¬ 
pation distance), and for which spectacles are most required. 

The prisms bases in measure the negative part of the 
range, and the prisms bases out its positive part. This is a 
very valuable test, but its value largely depends on the ratio 
that should exist between the negative and the positive parts, 
and then noting the departure from this normal ratio. This 
must be well worked out for different distances of vision; but, 
unfortunately, there is no such complete table to which the 
optician can refer. 

However, there are some data on the subject at hand 
which can be made use of. For instance, at twelve inches the 
negative and positive partis of the range of convergence are 


METHOD OF EXAMINATION. 267 

equal, while at shorter distances the positive part begins to 
exceed the negative. 

It is a fact that the normal relation existing between con¬ 
vergence and accommodation becomes altered temporarily by 
prolonged efforts to overcome strong prisms, and, therefore, 
all these tests should be made as quickly as possible. 

In testing the ranges of convergence with a single prism, 
it must be divided in half in order to get the result for each 
eye. Or, if a pair of prisms of unequal numbers are used, that 
is, one of the prisms weaker than the other, the sum of the two 
must be divided in half in order to' get the result for each eye. 
But, unquestionably, the best way is to use a pair of similar 
prisms, of the same degree before each eye. 

It should not be understood that there is any prism which 
measures definitely the normal strength of either pair of recti 
muscles. There are, indeed, in apparently normal eyes, re¬ 
markable variations in the ability to overcome prisms placed 
in the position required by the tests. A considerable number 
of observations seem to show that an emmetrope possesses a 
power of convergence of about 30°, and of divergence of 8°, 
the average ratio between the two being 100 to 28. 

A hypermetrope requires a greater amount of accommo¬ 
dation to overcome his defect, and, therefore, it might be ex¬ 
pected that his power of convergence would also be propor¬ 
tionately greater than with an emmetrope; but, on the 
contrary, it is usually less. In these cases the power of con¬ 
vergence is about 25 0 , and of divergence 12 0 , the average ratio 
between the two being as 100 to 48. 

NOMENCLATURE OF MUSCULAR ANOMALIES. 

Of late years a new nomenclature has been proposed and 
adopted for expressing degrees of muscular weakness, and as 
it is being used to a considerable extent by writers of books, 
and as the optician is apt to meet it in his reading, it is im¬ 
portant for him to familiarize himself with it. 

Orthophoria is the term used to denote parallelism of the 
visual lines or normal power of the muscles; a condition of the 
eyes where there is perfect balance and coordination of all 
the extrinsic ocular muscles. 


268 


METHOD OF EXAMINATION. 


Heterophoria is the term employed to denote some de¬ 
parture from the normal parallelism of the visual lines, and 
signifies that condition of the eyes where there is a want of bal¬ 
ance of the extrinsic ocular muscles due to insufficiency or 
paralysis of some one of these muscles. 

Esophoria, a convergence or tendency of the visual lines 
inward; or, insufficiency of the external recti muscles. 

Exophoria , a divergence or tendency of the visual lines 
outward; or, insufficiency of the internal recti muscles. 

Hyperphoria (right or left) is the term used to denote a 
tendency of the right or left visual line to place itself in a 
direction above that of the opposite side; or, insufficiency of 
the inferior rectus muscle. 

Cataphoria (right or left) is the term used to denote that 
the visual line of one eye is below that of its fellow; or, insuffi¬ 
ciency of the superior rectus muscle. 

COMPOUND TERMS. 

Tendencies of the eye to deviate in an oblique direction 
are expressed by the following terms: 

Hyper-esophoria signifies a tending of the visual line up¬ 
ward and inward; or, insufficiency of the inferior and external 
recti muscles. 

Hyper-exophoria signifies a tending of the visual line up¬ 
ward and outward; or, insufficiency of the inferior and internal 
recti muscles. 

Eso-cataphoria signifies a tending of the visual line inward 
and downward; or, insufficiency of the external and superior 
recti muscles. 

Exo-cataphoria signifies a tending of the visual line out¬ 
ward and downward; or, insufficiency of the internal and 
superior recti muscles. 

In these four latter forms of deviation, the oblique muscles 
are also frequently at fault. 

TESTS FOR MUSCULAR INSUFFICIENCY. 

In testing the strength of the muscles in order to detect 
any insufficiency thereof, the first step is to dissociate conver¬ 
gence and accommodation, or to produce vertical diplopia. 


METHOD OF EXAMINATION. 


269 


This is accomplished by a vertical prism (base either up or 
down), which removes the stimulus for singleness of vision, 
and if any muscular insufficiency is present, it will be made 
manifest by the overaction of its opposing muscle. A very 
weak prism will suffice for this purpose, because the inferior 
and superior recti muscles are able to overcome only a very 
small amount—not more than two or three degrees; hence, 
a prism a little stronger than this is recommended, and any 
prism from five to fifteen degrees may be used. 

The vertical prism is placed before one eye and the patient 
is directed, with both eyes open, to look at a distant object, say 
a candle flame at a distance of twenty feet or more. Vertical 
diplopia being produced by the prism, two images of the 
candle flame will be seen, one above the other. If the upper 
image is exactly above the lower, both being in the same ver¬ 
tical line without any muscular effort, the inference is that 
there is no insufficiency of either the internal or external recti 
muscles; or, in other words, these muscles balance each other. 

But if the two vertical images are not in the same straight 
line, muscular insufficiency is known to exist, and the relative 
positions of the two images will indicate the particular form of 
insufficiency, whether it be of the internal or the external recti 
muscles. 

If the prism be placed over the right eye with its base up, 
the image belonging to this eye will be the lower one. If, 
now, the lower image deviates to the right, the condition 
known as homonymous diplopia is present, which means that 
the right image is seen by the right eye and the left image by 
the left eye. This condition is due to an insufficiency of the 
external recti muscles (a condition of esophoria), and the de¬ 
gree of prism with its base out that is necessary to bring and 
retain the two images on the same vertical line the optician 
can accept as the measure of the amount of the insufficiency. 

If, on the other hand, the lower image deviates to the left, 
the condition present would be known as crossed diplopia, in 
which the right image belongs to the left eye and the left image 
to the right eye. This crossing of the images is due to an 
insufficiency of the internal recti muscles, which, according to 
the nomenclature as given above, would be exophoria. The 


METHOD OF EXAMINATION. 


270 

degree of prism with its base in 'that is necessary to bring and 
retain the two images on the same vertical line will indicate 
the amount of the insufficiency. 

DOT AND LINE TEST. 

The test should then be repeated for near objects by 
means of the dot and line, in order to detect any departure from 
the normal equilibrium of the internal or external muscles. 


Every optician is more or less familiar with this figure 
and its method of use. A prism is placed over one eye with its 
base vertical. The patient is asked to look at the above figure, 
which is drawn on a card, and held in the hand at a distance 
of twelve to fifteen inches. If the patient says he sees two 
dots on one line, the muscles are assumed to be normal. If he 
sees two dots and two lines laterally displaced, there is insuffi¬ 
ciency; and the direction of the displacement will indicate 
whether it is the internal or the external recti muscles that are 
at fault. 

If the lower image is on the same side as the eye over 
which the prism is placed (homonymous diplopia), the insuffi¬ 
ciency is in the external recti and is corrected by a prism base 
out. If the lower image is on the opposite side from the eye 
over which the prism is placed (crossed diplopia), the insuffi¬ 
ciency is in the internal recti muscles, and is to be corrected 
by a prism base in. 




METHOD OF EXAMINATION. 


271 


MADDOX TESTS. 

Maddox, an English oculist, has given the optical world 
several very valuable tests for the detection of muscular insuf¬ 
ficiencies. The Maddox Rod consists of a glass rod or cylinder 
(usually of colored glass and preferably red) set in a stenopaic 
opening of a metal disk. The Maddox Groove is a mounted 
lens of red glass, translucent, but not transparent, with a trans¬ 
parent groove ground across its equator. 

These lenses (both the rod and the groove) act as strong 
cylinders, and elongate the candle flame or gas-light looked at 
into a long, narrow streak of light, running at right angles to 
ithe direction in which the cylinder is placed before the eye. 



The Maddox Rod. 


This apparent lengthening of a flame into a line of light, 
when looked at through such a strong cylindrical lens by one 
eye, causes such a dissimilarity between the two retinal images 
as to destroy all desire to unite them and preserve binocular 



The Maddox Groove. 
















































272 


METHOD OF EXAMINATION. 


vision, and this fact is utilized as the basis on which the Mad¬ 
dox tests are founded. One of the advantages of this principle 
is that extremely great care is not required to see that the cyl¬ 
inder is exactly vertical or exactly horizontal, because slight 
malpositions of the cylinder do not materially vitiate the 
result, as is the case when testing with prisms. 

In making the test, the Maddox rod or groove is placed 
over one eye in the trial-frame, while the other eye is either left 
uncovered, or a light blue glass may be used before this eye in 
order to more nearly equalize the illumination of the two 
images. The rod or groove is placed in the trial-frame in 
either a horizontal or a vertical position, which, it should be re¬ 
membered, will cause the flame of light to be elongated into a 
streak of light and appear in either the vertical or horizontal 
meridian; while the image formed in the other eye will be that 
of the unaltered flame, except as its intensity of illumination 
may be modified by the colored lens. 

On account of the great difference in size, shape and ap¬ 
pearance of the two images as formed on the retina of each 
eye, binocular vision is destroyed, and, consequently, there is 
no tendency or effort to fuse the two images, and this allows 
each eye to take its natural position of rest; or, in other words,, 
gives the eyes over to the natural control each of its own mus¬ 
cles, uninfluenced by the desire that exists in every man’s 
mind to preserve binocular vision. This desire for binocular 
vision is naturally so strong that great muscular insufficiencies 
are oftentimes overcome and masked by the effort to maintain 
it. The removal of this desire, or the placing of the eyes each 
under the free control of its own muscles, will reveal any exist¬ 
ing insufficiency of any of them. If, on the removal of this 
natural stimulus for binocular vision, the eyes still remain 
straight, and both eyes equally directed to the object, it is safe 
to assume that there is no insufficiency of any of the ocular 
muscles; if, on the other hand, there is a deviation of one or 
both eyes, or a separation of the two images as formed in each 
eye, it is proof of the existence of insufficiency of one or more 
of the ocular muscles. 

The best flame is that of a gas jet turned low at a distance 
of fifteen or twenty feet, which the patient is directed to look 


METHOD OF EXAMINATION. 


273 


at with both eyes open through the trial-frame, which had 
been fitted with the rod and the colored glass, as above de¬ 
scribed. If orthophoria exists, that is, if there is a perfect bal¬ 
ance and coordination of all the ocular muscles, the streak of 
light seen by the eye over which the rod is placed will pass 
directly through the flame as seen by the other eye, and they 
will both occupy the same position. If, however, the streak 
of light is to one side or the other of the flame, hcterophoria is 
known to exist, that is, a departure from the normal parallel¬ 
ism of the visual lines. 

If any marked error of refraction is present in the eye that 
is being examined, it should be corrected by the proper lens, 
after which the glass rod should be placed over the right eye, 
if it is desired to examine the muscles of this eye first. The 
flame is then seen in its natural condition by the left eye and is 
fixed by that eye, while the streak of light seen by the other 
eye will be influenced and thrown to one side or the other, 
according to the strength or weakness of the internal or ex¬ 
ternal recti muscles of this eye. The glass rod is placed before 
the eye in the horizontal meridian when it is desired to ex- 


A 


amine the internal and external recti muscles (or the adductors 
and abductors, as they are sometimes called), for the reason 
that with the rod in this position the line of light is vertical, 
and a lateral deviation of a vertical line could more quickly be 
noticed than could a perpendicular deviation; or, in other 
words, the line should be at right angles to the deviation 
which it is desired to measure. 











METHOD OF EXAMINATION. 


274 


The flame of light being fixed by the left eye, any muscu¬ 
lar insufficiency of the right eye will be made manifest by the 
line of light placing itself to one side or other of the flame. 
If the line appears on the right 'side, that is, on the same side 
as the eye that is being examined, it shows the existence of 
esophoria, or an insufficiency of the external recti muscles. 
The degree of esophoria is measured by the strength of prism, 
base out, that is required to bring the line of light directly 
over the flame. This form of diplopia is known as homon¬ 
ymous. 

If the line of light appears on the left side, that is, on the 
opposite side from the eye that is being examined, it shows the 
existence of exophoria, or an insufficiency of the internal recti 
muscles. The degree of exophoria is measured by the strength 
of prism, base in, that is required to bring the line of light 
directly over the candle flame. This form of diplopia is known 
as heteronymous, or crossed, and is the most common form. 

The glass rod can then be placed over the left eye and the 
tests repeated in the same way, and it will be found if there had 
been esophoria of the right eye, there will also be esophoria of 
the left, that is, if the insufficiency of the muscles of the right 
eye had been seated in the external rectus, it will be found in 
the same muscle of the left eye. Likewise, if there had been 
exophoria of the right eye, there will also be exophoria of the 
left, that is, if the internal rectus muscle of the right eye is 
weak, the internal rectus of the left eye will be affected in the 
same form and degree. 

In the first case the correction is made by a prism, base 
out, the whole amount of which can be placed over one eye, 
or the prism can be divided and half placed over each eye. In 
the second case the correction is made by a prism, base in, 
which can be placed over either eye, indifferently, or divided 
between the two eyes. 

In the examination of the superior and inferior recti 
muscles of the eyes, the glass rod is placed in the trial-frame 
in a vertical position. This causes the line of light to be seen 
horizontally, and the easier permits a perpendicular deviation 
to be detected, for the reasons given above. 

The rod is first placed over the right eye and a light- 


METHOD OF EXAMINATION. 


275 


colored glass over the left eye, both lenses being in the trial- 
frame, and again the patient is directed to look with both eyes 
at the gas jet fifteen or twenty feet away. The flame is seen 
in its normal shape and is fixed by the left eye, while the image 
formed in the right eye is a horizontal line of light, which will 
either pass through or over the flame, or be placed above or 
below it, according as these muscles are normal or abnormal 
in strength. 

If the line of light is identical with the flame, the superior 
and inferior recti muscles are assumed to be normal, which 
condition is expressed by the term orthophoria. If the streak 
of light is seen below the flame, this proves an insufficiency of 



the inferior rectus muscle of this eye; this condition is known 
as hyperphoria, which means a tendency of this eye to place 
its visual line in a direction above that of its fellow, which, 
when projected outward, causes the image to be seen below. 
The degree of the hyperphoria is measured by the strength of 
the prism, base down, which is required to place the line of 
light directly over the flame. 

If the line of light is seen above the flame, there is an in¬ 
sufficiency of the superior rectus muscle of this eye, which 
condition is known as cataphoria, which is a term used to de¬ 
note that the visual line of this eye is below that of the other, 
the projection of which outward causes the image to be seen 
above. The degree of the cataphoria is measured by the 
strength of the prism, base up, which is required to place the 
line of light directly over the flame. 

COMPOUND MUSCULAR DEFECTS. 

In addition to the tests just given for measuring the 
strength of the internal and external recti muscles, and of the 











276 


METHOD OF EXAMINATION. 


inferior and superior recti muscles, Maddox has introduced 
another test to discover any compound muscular error, as by 
its use can be ascertained at once any defect of the vertical 
muscles with that of the horizontal muscles, as well as of the 
oblique muscles. 

This is accomplished by his double prism test, which con¬ 
sists of two prisms set together in a rim with their bases 
together, and which has the effect of making objects appear 
double. 

In the practical application of this test, the metal rim 
enclosing the double prism is placed in the trial-frame over 
one eye, while the other eye is excluded from vision by the 
opaque disk. Care must be taken to see that the line of sepa¬ 
ration between the two prisms shall be directly in front of the 
center of the pupil, which will have the effect of forming tw T o 
images in this eye and making any object looked at appear 
double. If preferred, the trial-rim may be held in the hand 
before the eye. In each case the dividing line between the 
prisms is to be placed horizontally. 

The patient is asked to look at any conspicuous object, 
as a door-knob or a candle flame, at a distance of fifteen or 
twenty feet. The double prism may be placed over the right 
eye and the metal disk over the left eye, in which case the pic- 



Maddox’s Double Prism. 


ture formed in the right eye will be two images of the object 
separated vertically quite a little distance (see Fig. 1, page 
277.) 



METHOD OF EXAMINATION. 


277 


If, now, the left eye be uncovered by the removal of the 
opaque disk, a third image of the object will be seen, as the 
picture formed in this eye is a single image of the object unin¬ 
fluenced by any disk or lens; and if orthophoria exists, this 
third image (belonging to the left eye) will be midway between 
the two images of the right eye and in the same vertical line, 


9 • 

9 

* • 

Fig. 1. Fig. 2. 


• 9 

® ® 

© 0 

Fig. 3. Fig. 4. 


• 0 

9 9 

Fig. 5. Fig. 6. 


as represented in Fig. 2. This, then, is the test for detecting 
any insufficiency of the internal or external recti muscles, and 
the fact that the three images are seen on the same straight 
vertical line indicates that these muscles are normal. 

The middle image must be not only on the same vertical 
line in orthophoria, but must also be equidistant from the 
upper and lower images. If the middle image deviates from 
this exact central position, and approaches either the upper or 
lower image, it indicates the existence of some insufficiency 
either of the superior or inferior reobi muscles. 

As just mentioned, the deviation of the middle object 
from the straight vertical line indicates an insufficiency either 
of the internal or external recti muscles, and the direction of 
the deviation will determine which of the muscles is affected. 
If the middle image is seen to the right (as in Fig. 3), it indi¬ 
cates exophoria, or an insufficiency of the internal rectus mus¬ 
cle. A little thought will show the student that this is a con¬ 
dition of heteronymous, or crossed, diplopia, the right image 
of the middle dot belonging to the left eye, and the left image 
of the upper and lower dots belonging to the right eye. This 
condition is corrected by a prism, base in, and the measure of 
the insufficiency will be that degree of prism, base in, over the 
left eye which throws the middle image in on the same ver¬ 
tical line as the other two. 

If the middle image appears to the left (as in Fig. 4), it 


278 


METHOD OF EXAMINATION. 


indicates esophoria, or an insufficiency of the external rectus 
muscle. This corresponds to a condition of homonymous 
diplopia, in which the right image of the upper and lower dots 
is seen by the right eye, and the left image of the single middle 
dot is seen by the left eye. This condition is corrected by a 
prism, base out, and the measure of the insufficiency of the 
external rectus muscle will be that degree of prism, base out, 
over the left eye which throws the middle object out on the 
same vertical line with the upper and lower images. 

If the middle image appears to move up and approaches 
the upper image (as in Fig. 5), it indicates cataphoria of the 
left eye, or, an insufficiency of the superior rectus muscle of 
this eye. In this condition the antagonistic muscle, the in¬ 
ferior rectus, being the stronger, draws the eye down and 
places its visual line in a direction below that of its fellow, 
in which position the projection of the image outward throws 
it up, because the image is formed on the retina below the 
yellow spot, on account of the fundus being moved upward 
by the downward direction of the front of the eye. This con¬ 
dition is corrected by a prism, base up, and the measure of the 
insufficiency of the superior rectus muscle will be that degree 
of prism with its base up over the left eye which throws the 
image down to its proper position, midway between the upper 
and lower ones. 

If the middle image appears to move down and ap¬ 
proaches the lower (as in Fig. 6), it indicates hyperphoria of 
the left eye, or, an insufficiency of the inferior rectus muscle of 
this eye. In this condition the antagonistic muscle, the 
superior rectus, being the stronger, draws the eye up and 
places its visual line in a direction above that of its fellow. 
When the eye is thus turned up the fundus of the eye is 
thrown down, and, consequently, the image is formed on the 
retina above the yellow spot, and, when projected outward, is 
seen below its normal position. This condition is corrected 
by a prism, base down, and the measure of the insufficiency of 
the inferior rectus muscle will be that degree of prism with its 
base down over the left eye which will cause the image to 
move upward and assume its proper position, midway between 
the upper and lower images. 


METHOD OF EXAMINATION. 279 

In these measurements the correcting prism is always 
placed over the left eye, because it is the image of this eye that 
deviates. The conditions may be reversed and the double 
prism placed over the left eye, in which case it will be the 
image of the right eye that deviates, and then the correcting 
prism will be placed over the right eye. It should be noted 
that the base of the correcting prism is always placed over 
the insufficient muscle; or, in other words, the apex of the 
prism is in the same direction in which it is desired to move 
the middle object. 

These tests, with their resulting deviations, will bear close 
study. No man can carelessly read them over and gather 
their full meaning, but the optician who desires to understand 
this somewhat intricate subject of muscular troubles must re¬ 
read these tests and study them closely, and he will see that 
there is a fixed principle that governs and explains them all; 
and when this is understood and becomes clear to the mind, 
the whole subject, that formerly seemed difficult and mixed, 
will open up clearly and beautifully. 

COMPOUND INSUFFICIENCY. 

If the middle image is seen to the left and above (as .shown 
in Fig. 7), it indicates a condition of eso-cataphoria, which 
means an insufficiency of both the external and superior recti 
muscles of the left eye. In this condition the antagonistic 








• • 

Fig. 7. Fig. 8. 


• • 

Fig. 9. Fig. 10. 


muscles, the internal and inferior recti, being the stronger, 
draw the eye-ball downward and inward, and place its visual 
line in a direction below and to the inside of its fellow. When 


METHOD OF EXAMINATION. 


280 

the eye is thus turned downward and inward, the fundus of the 
eye is placed upward and outward, and, consequently, the 
image is formed on the retina below the yellow spot and to the 
inside of it, and, when projected outward, is seen above and to 
the left of its normal position. 

If the middle image is seen to the right and above (as 
shown in Fig. 8), it indicates a condition of exo-cataphoria, or, 
an insufficiency of both the internal and superior recti muscles 
of the left eye. Under these circumstances, the antagonistic 
muscles, the external and inferior recti, being the stronger, 
draw the eye-ball outward and downward, and place its visual 
line in a direction below and to the outside of its fellow. In 
this position the fundus of the eye is turned upward and in¬ 
ward, and, consequently, the image is formed on the retina 
below the yellow spot and to the outside of it, and, when pro¬ 
jected from the eye, is seen above and to the right of its normal 
position. 

If the middle image appears to the left and below (as 
shown in Fig. 9), it indicates a condition of hyper-esophoria, 
or, an insufficiency of both the external and inferior recti mus¬ 
cles of the left eye; in which case the antagonistic muscles, the 
internal and superior recti, being the stronger, draw the eye¬ 
ball inward and upward, and place its visual line in a direction 
above and to the inside of its companion. In this position, with 
the fundus of the eye turned downward and outward, the 
image is formed on the retina above and to the inside of the 
yellow spot, and, when projected out from the eye, appears 
below and to the left of its normal position. 

If the middle image appears below and to the right (as 
shown in Fig. 10), it indicates a condition of hyper-exophoria, 
or, an insufficiency of both the internal and inferior recti mus¬ 
cles, in which case the antagonistic muscles, the external and 
superior recti, draw the ball outward and upward, and place 
its visual line in a direction above and to the outside of the 
other eye. In this position, the fundus of the eye being turned 
downward and inward, the image is formed on the retina 
above the yellow spot and to the outside of it, and, when pro¬ 
jected out from the eye, appears below and to the inside of its 
normal position. 


METHOD OF EXAMINATION. 


28l 

DETECTION OF INSUFFICIENCY OF THE OBLIQUE MUSCLES. 

In the detection of insufficiencies of the oblique muscles, 
Maddox double prism is again brought into use, and is placed 


Fig. 11. 

over one eye, as in the preceding tests, while the other eye is 
covered by the opaque disk. The patient is asked to look at a 
card, which is held in his hand at a distance of eighteen inches, 
and which contains nothing but a plain horizontal line (as in 
Fig. 11). He will at once see two lines parallel with each 


Fig. 12. 

other (as in Fig. 12), because the effect of the double prism is 
to produce double vision, or vertical diplopia, if the prism is 
placed before the eye in isuch a way that its prisms are vertical 
and the line of separation horizontal and directly in front of 
the center of the pupil. 

The other eye is now uncovered by the removal of the 
opaque disk, when a third line is seen between the other two; 






282 


METHOD OF EXAMINATION. 


Fig. 13. 

if the muscles are normal, this third line should be equidistant 
between the two and parallel with them (as in Fig. 13). If 
there is any insufficiency or want of harmony on the part of 
the oblique muscles, this test will at once detect it by a de¬ 
parture of the middle line from its parallelism with the other 
two lines, the one end of the middle line pointing up and the 
other down, or vice versa , according to the nature of the par¬ 
ticular case. 

When it is desired to test the muscles of the right eye, 
which is usually done first, the double prism is placed over the 
left eye, and the patient’s attention is directed to the middle 
line. This middle line is seen by and belongs to the right eye, 
and will depart from its normal position if there is any insuffi¬ 
ciency of the oblique muscles, while the upper and lower lines, 
which are seen by and belong to the other eye, which is not 
under examination, always remain stationary. 

We will suppose that the patient says the right end of 
middle line slants toward the bottom line, and that the middle 



and lower lines seem to converge at the right and diverge at 
the left (as in Fig. 14); what is the optician to understand from 
this? Does this deviation tell him, in unmistakable language, 







METHOD OF EXAMINATION. 283. 

which muscle is affected, and whether it is too weak or too 
strong? or what significance does it convey to his mind? 

In the study of these deviations, the optician should re¬ 
fresh his memory as to the origin and insertion and function of 
the oblique muscles. The superior oblique muscle is placed at 
the upper and inner side of the orbit. It has its origin at the 
apex of the orbit, and as it passes forward to the inner angle of 
the orbit it terminates in a rounded tendon, which plays in a 
ring or pulley, formed by fibro-cartilaginous tissue attached to 
a depression on the frontal bone. This tendon passes outward 
and backward beneath the superior rectus muscle to the outer 
part of the globe of the eye, and is inserted in sclerotic coat 
midway between the cornea and the entrance of the optic 
nerve, and between the superior and external recti muscles. 

The inferior oblique muscle is a thin, narrow muscle, and 
placed near the anterior margin of the orbit. It takes its origin 
from the anterior and inferior portion of the orbit, and passes 
outward and backward beneath the inferior rectus muscle and 
between the eye-ball and the external rectus, and is inserted 
into the outer part of the sclerotic coat between the superior 
and external recti muscles, and near the tendon of insertion oi 
the superior oblique. 

The oblique muscles rotate the eye-ball on its antero¬ 
posterior axis, this kind of movement being required for the 
correct viewing of an object when the head is moved laterally, 
as from shoulder to shoulder, in order that the picture may fall 
in all respects on corresponding parts of the retina of each eye. 
The superior oblique mu c cle rotates the eye-ball slightly 
downward and outward; while the inferior oblique muscle 
rotates the ball slightly upward and outward. Or, in other 
words, the action of the superior oblique muscle is to rotate 
the eye-ball on its axis in such a way as to turn the superior 
portion inward and the inferior portion outward; while the 
action of the inferior oblique muscle is to rotate the superior 
portion of the eye-ball outward and the inferior portion 
inward. 

OBLIQUE INSUFFICIENCIES. 

Therefore, when the optician tests the right eye and finds 
the middle line slanting in such a way as is shown in Fig. 14, 


284 


METHOD OF EXAMINATION. 


he knows there is weakness or insufficiency of the inferior 
oblique muscle of this eye, which allows -the eye to be rotated 
in this direction. 

If, however, the patient says the right end of the middle 
line slants toward the top line, and that the middle and upper 
lines seem to converge at the right and diverge at the left (as 
Fig. 15), the optician is able by the same line of reasoning to 
conclude that in this case there is a weakness of the superior 
oblique muscle of the right eye. 



After having tested the oblique muscles of the right eye 
in this way, the optician removes the Maddox double prism 
from the left eye and places it over the right eye, and then he 
is in a position to examine the oblique muscles of the left eye, 
as the -image of the middle line now belongs to this eye. If 
now the patient says he sees the right end of the middle line 
slanting toward the bottom line, and that the middle and lower 
lines seem to converge at the right and diverge at the left (as 
in Fig. 14), the optician knows that the superior oblique 
muscle of this left eye is below the normal strength. 

If, on the other hand, the patient says the right end of the 
middle line slants toward the top line, and that the middle 
line and top line seem (to converge at the right and diverge at 
the left (as in Fig. 15), -the optician recognizes this as a case 
of insufficiency of the inferior oblique muscle of this left eye. 

A careful study of these tests and diagrams will show that 
the inferior oblique muscle of the right eye acts in conjunc¬ 
tion with the superior oblique of the left eye; as the upper 
portion of the right eye rotates outward by the action of its 
inferior oblique muscle, the upper portion of the left eye fol¬ 
lows it by rotating inward by the action of its superior oblique 




METHOD OF EXAMINATION. 


285 


muscle. And, therefore, an insufficiency of the inferior oblique 
muscle of the right eye will be detected by the same test and 
result as an insufficiency of the superior oblique muscle of the 
left eye (as in Fig. 14). 

On the other hand, the superior oblique muscle of the 
right eye acts in conjunction with the inferior oblique muscle 
of the left eye; as the upper portion of the right eye rotates on 
its axis inward by the action of the superior oblique muscle, 
the upper portion of the le'ft eye follows it by rotating outward 
on its axis by the action of its inferior oblique muscle. And, 
therefore, a weakness of the superior oblique muscle of the 
right eye can be detected by the same test and result as an 
insufficiency of the inferior oblique muscle of the left eye (as 
in Fig. 15). 


CHROMATIC TEST. 

This is a corroborative test for the errors of refraction, 
and depends upon chromatic aberration, or the difference in 

refraction of different colored 
rays of light. If the reader will, 
refer to the subject of chromatic 
aberration on page 78, and 
the diagram of the separation of 
colors on page 79, he will see 
the explanation of the principle 
on which this test is based. 
It should be constantly borne in 
mind that the red rays are the 
least deviated from their original 
course, and the blue and violet 
rays the most. 

The chromatic lens used in this test is made of cobalt 
blue; it suppresses all the colors except the two extremes, 
allowing only the red and blue rays to pass. The flame of a 
lamp or candle is used at a distance of fifteen or twenty feet r 
and is viewed through the chromatic lens, when the blue rays, 
being the most strongly refracted, will come to an earlier focus 
and the red rays, being the least refracted, will meet at a later 
focus. 



286 


METHOD OF EXAMINATION. 


If the eye is emmetropic, the focus of the blue rays will be 
as far in front of the retina as the focus of the red rays is 
behind it, and, consequently, the two sets of rays will intersect 
at the position of the retina, and this intermingling of the two 
colors at this point will cause the flame to appear as of a 
diffuse violet color with a border of a slightly deeper hue. 

If the eye is hypermetropic, the retina will be advanced and 
will approach the focus of the blue rays (as is shown in the 
diagram), and this will cause the flame to appear with a blue 
center and a red border. 

In a myopic eye, on the other hand, the position of the 
retina is farther back, in proximity to the focus of the red rays 
(as illustrated in the diagram), and in this case the flame will 
appear with a red center and a blue border. 

To determine the degree of defect we find that convex or 
concave lens that will correct the chromatic aberration. In 
hypermetropia we prescribe the convex lens that will dispel 
the red border and convert the light into a violet red. In 
myopia we find the concave lens that will diffuse the red 
center and blue border into one. 

In hypermetropic astigmatism the flame, instead of being 
circular, will appear elongated, with a blue center and red ex¬ 
tremities, and the convex cylinder that will restore the flame 
to its circular shape will be the measure of the astigmatism. 

In myopic astigmatism the flame will also appear elongated, 
with a red center and blue extremities, and the concave lens 
that removes the elongation will be the measure of the defect. 

In this diagram we see the red and blue rays in a parallel 
condition approaching and entering the eye; the blue rays, 
being more strongly refracted, meet in a focus just behind the 
line H, and the red rays, being least refracted, meet just before 
the line M . The line H indicates the position of the hyperme¬ 
tropic retina, where the eye perceives a blue center and a red 
outline H', the latter being formed by the red rays, which have 
not yet come to a focus. 

The line M indicates the location of the myopic retina, 
where the eye perceives a red spot (the focus of the red rays) 
with a blue border (see AT), formed by the rays which had 
previously met and crossed. The line E indicates the position 







I 



/ 

































































































' 













1 








METHOD OF EXAMINATION. 287 

of the retina of the emmetropic eye, where the red and blue 
rays cross each other and form the diffused violet tint E. 

In emmetropia the light will appear of a diffuse violet 
tint, as shown by A; in hypermetropia, of a blue center and a 
red border, as shown by B; and in myopia, of a red center and 
a blue border, as shown by C. 

In simple hypermetropic astigmatism the flame appears 
elongated, with a blue center and red extremities, as shown by 
D, or a cross, as shown at E. In simple myopic astigmatism 
the elongated figure has a red center and blue extremities, as 
shown at F, or a cross, as shown at G. The appearance of the 
flame in mixed astigmatism is shown by H and /, and in 
irregular astigmatism by / and K. 

recapitulation: how to examine the eye systemat¬ 
ically. 

This ninth chapter on the “Method of Examination” of 
the eye is a very lengthy one. It covers a wide field and em¬ 
braces many matters of importance in the detection and cor¬ 
rection of the various errors of refraction and anomalies of the 
muscles of the eye. In drawing this chapter to a close, it will 
t>e of advantage to the student to briefly rehearse the import¬ 
ant points or steps to be taken in a systematic and thorough 
examination of the eye, or, in other words, present a brief 
outline and recapitulation of the chapter. 

“It is said that any one can administer treatment in dis¬ 
ease if he knows the disease, and ‘there’s the rub,’ for to know 
the disease—in other words, to correctly diagnosticate—one 
needs not only to be familiar with the symptoms in their vari¬ 
ous forms, but he must carefully examine; and to examine 
carefully he must not only have the necessary means of exami¬ 
nation and various tests, but he must know how to employ 
them.” 

A complete examination of the eye embraces both sub¬ 
jective and objective methods: The former refers to the infor¬ 
mation gained from the patient’s answers, while the latter 
refers to the information gained by the optician himself from 
an inspection of the conditions that are present and are appre- 


288 


METHOD OF EXAMINATION. 


ciable to his educated senses, which includes an inspection of 
the head and face, the appearance of the eye, whether full or 
shallow, whether any difference in the two eyes or any asym¬ 
metry of the face, the size and dilatability of the pupils, and 
whether any convergence or divergence of the eyes exists. 

The condition of the conjunctiva, of the cornea, of the 
aqueous humor, of the iris and pupil, are all noted in turn; 
after which the subjective examination is taken up by interro¬ 
gating the patient as to his symptoms and obtaining the his¬ 
tory of the case. 

The next step is to use the pin-hole disk in order to be 
able to determine whether the case is one of refractive error or 
organic disease of the eye. This is a simple and reliable 
method of diagnosis: If the disk improves vision, it is a case 
for glasses; if the disk fails to benefit the sight, the eye is 
beyond the optician’s help. 

Next in order comes the determination of the acuteness of 
vision, which is accomplished by means of Snellen’s test- 
letters, and is expressed by a fraction, the numerator of which 
denotes the distance at which the card is placed, and the de¬ 
nominator the number of the line which it is possible to read. 
As, for example, if the card is hanging at a distance of twenty 
feet, and the patient is able to read the No. 20 line, the acute¬ 
ness of vision is expressed as follows: V. = ■§-§■. If only the 
No. 30 line or the No. 40 line can be read, then V. = or 
V- = T¥ • I't is desirable to have the test-card hanging at a 
distance of twenty feet in order to exclude as much as possible 
any effort of accommodation, as the refraction of the eye and 
the acuteness of vision are both determined at the same time. 

The acuteness of vision, as expressed above, has reference 
to direct vision, or the vision pertaining to the yellow spot; in 
addition to which it is sometimes desirable to determine the 
indirect vision or that pertaining to the peripheral portions of 
the retina, which is best accomplished by means of a perime¬ 
ter. The quantitative and the qualitative field of vision can 
be determined, and the extreme limits of the field of vision 
mapped out, and the existence of any possible blind spots can 
be discovered. As the different meridians of the eye are ex¬ 
amined, the record can be transferred to a sheet of paper, and 


METHOD OF EXAMINATION. 289 

a diagram will thus be formed, showing the size and shape of 
the visual field. 

The examination of direct vision in the emmetropic eye 
has shown it to be equal to J--J . With vision as good as this, 
hypermetropia may possibly exist, but myopia and astigma¬ 
tism may reasonably be excluded. Therefore, when the optician 
finds a case with a vision of |-J, the question involuntarily 
arises in his mind, Is this a case of emmetropia, or is there 
any element of hypermetropia present in the case? This is to 
be determined by the acceptance or non-acceptance of convex 
lenses; the trial is first made with a very weak convex spherical 
lens, which is gradually increased in strength as the patient 
will bear, until the strongest lens is found which improves 
distant vision—or, at least, does not blur distant vision—and 
this lens will be the measure of the manifest hypermetropia. 
If all convex lenses are rejected, even the weakest (+ .25), the 
optician may reasonably conclude that the case is one of em¬ 
metropia. 

If the patient’s vision is found to be less than f§, it is due 
either to some error of refraction or to some organic disease. 
The pin-hole disk has already been used to determine this 
latter point. If due to a refractive error, it may be either 
hypermetropia, myopia or astigmatism. The test with convex 
lenses will determine whether or not the case is one of hyper¬ 
metropia. If this is excluded, the optician next proceeds to 
determine whether myopia is present, which is done by testing 
the eyes with concave lenses. The restoration of the patient’s 
vision to the normal standard by concave glasses proves the 
case to be one of myopia, and the weakest concave lens that 
affords a vision of -f# will be the measure of the myopia. 

If concave lenses do not satisfactorily improve the vision, 
and if hypermetropia and myopia are excluded, the next 
thought of the optician will be as to the existence of astigma¬ 
tism. This is usually determined by means of Pray’s card of 
astigmatic letters, or by the card of radiating lines; if some of 
the letters and some of the lines appear very much blacker and 
clearer than others, it is proof of the existence of astigmatism, 
which is then measured and corrected by cylindrical lenses. 

If the defective vision is not due to any error of refraction, 


290 


METHOD OF EXAMINATION. 


and if the optician is unable to improve the vision by any glass, 
then an ophthalmoscope comes into play very nicely and ap¬ 
propriately to determine the seat and cause of the impaired 
vision, and, therefore, no optician’s outfit can be considered 
complete unless it embraces this useful instrument, which for¬ 
merly was reserved for physicians’ use alone. 

Normal Vision, or j f Emmetropia, or, possibly, Hy- 

a vision of f£, I means ( permetropia. 

S Hypermetropia. 

Myopia. 

Astigmatism. 

Amblyopia. 

Spasm of Accommodation. 
Cataract. 

Opacity of Cornea. 

Organic Disease of some of 
the Humors or Membranes. 

Next in order comes the examination of the near vision, 
or the testing of the accommodation, or the measuring of the 
near point and far point. The nearest point at which the read¬ 
ing matter can be distinguished, that is, the closest point for 
which the eye can accommodate itself, is called the near point. 
The greatest distance at which the same type can be read 
may be considered for all practical purposes as the far point , 
although, strictly speaking, in emmetropia this is said to be 
at infinity. The distance between the near point and far point 
is called the range of accommodation. The force necessary to 
change the eye from its far point to its near point is called the 
amplitude of accommodation. 

The accommodation is equal to a convex lens of such 
strength as would give to rays proceeding from the near point 
a direction as if they came from the far point, and in emme¬ 
tropia the focus of this lens coincides with the distance of the 
near point. Therefore, in order to determine the amplitude of 
accommodation of the emmetropic eye, the optician has only 
to find the nearest point at which the patient is able to read 


METHOD OF EXAMINATION. 


291 


the smallest sized type. It may also be determined by a con¬ 
cave lens, as the strongest concave lens through which an em¬ 
metropic eye is still able to see clearly at a distance is the 
measure of the amplitude of its accommodation. 

In hypermetropia, the amplitude of accommodation 
which would be normally present in an emmetropic eye will be 
increased by the amount of hypermetropia, and in myopia it 
will be diminished by the degree of the defect that is present. 

Having measured the refraction and the accommodation, 
the optician now directs his attention to the function of con¬ 
vergence, which, it will be remembered, bears a constant rela¬ 
tion to the accommodation. The function of convergence has 
reference to the directing of the two eyes to a single point 
which is situated nearer than infinity, and the angle of con¬ 
vergence is sometimes called the meter angle, when the eves 
are directed to a point situated at a distance of one meter. 
This expresses the degree of convergence which is required to 
maintain binocular vision at this distance, and is employed as 
a unit from which to express other degrees of convergence. 
In this case the metrical angle equals one, and is expressed, 
C. = 1. 

If the object looked at be situated at half a meter dis¬ 
tance, the angle of convergence will be twice as large as when 
at one meter, and is expressed or written, C. = 2. If at one- 
third a meter, the angle of convergence is correspondingly 
increased, and then, C. = 3. If, on the other hand, the object 
be situated at a greater distance than one meter, say at two 
meters or four meters, the angle of convergence will be dimin¬ 
ished in proportion, and then we have, C. = J, or, C. = The 
angle of convergence diminishes in this same proportion as the 
distance of the object is increased, until finally, when infinity 
is reached, the angle of convergence will have disappeared and 
the visual lines become parallel. 

In emmetropia, for distances nearer than twenty feet, the 
number of meter angles of convergence required bears a con¬ 
stant relation to the number of dioptres of accommodation 
called for. At a distance of one meter, 1 D. of accommodation 
is required to focus the image on the retina; and at the same 
distance one meter angle of convergence is needed for binocu- 


292 


METHOD OF EXAMINATION. 


lar vision. At a distance of half a meter, there is 2 D. of ac¬ 
commodation and two meter angles of convergence. 

TESTING MUSCULAR INSUFFICIENCY. 

The function of convergence being dependent on the 
action of the internal recti muscles, it becomes an important 
matter to know how to determine the strength of these mus¬ 
cles, and also to detect any weakness of them and of the other 
ocular muscles. This is best accomplished by testing the 
muscles with prisms. When a prism is held before one eye, 
the eye will have to turn in the opposite direction from which 
the base of the prism is placed in order to preserve binocular 
vision. 

The power of divergence, or the strength of the external 
recti muscles, is measured by the strongest pair of prisms, 
bases in, which will not cause double vision of a distant object. 
The normal average power of the external recti muscles is 
represented by a prism of from 8° to io°. 

The power of convergence, or the absolute maximum of 
convergence, or the strength of the internal recti muscles, is 
determined by finding the strongest prism over one eye, or the 
strongest pair of prisms over both eyes, bases out, which will 
not destroy single vision of an object held as close to the eyes 
as the accommodation will permit. The normal power of con¬ 
vergence, or the average strength of the internal recti muscles,, 
is represented by a prism of from 20° to 30°. 

In testing the ocular muscles, in order to detect any in¬ 
sufficiency, the first step is to dissociate the functions of con¬ 
vergence and accommodation, which is accomplished by a 
prism placed in a vertical position. A very weak prism will 
suffice for this purpose (2 0 or 3 0 ), and when vertical diplopia is 
produced in this way, any insufficiency of the internal or ex¬ 
ternal recti muscles becomes apparent by a lateral displace¬ 
ment of one of the images. 

In testing the muscles at a distance of twenty feet or 
more, a candle flame is used; at a near distance, the dot and 
line is to be preferred. It is customary to use a prism of io°, 
which is placed over one eye with its base up or down; vertical 
diplopia is at once produced, and if the two images are in the 


METHOD OF EXAMINATION. 


293 


same vertical plane, the muscles are assumed to be of normal 
strength. But if there is a lateral deviation, it is due to the 
existence of muscular insufficiency. If the diplopia is ho¬ 
monymous, the insufficiency is located in the external recti 
muscles, a condition of esophoria. If the diplopia is crossed, 
the insufficiency is located in the internal recti muscles, a con¬ 
dition of exophoria. In the first case, the degree of prism, base 
out, and in the second case, the degree of prism, base in, neces¬ 
sary to restore the images to the same vertical line are the 
measure of the insufficiency. 

This test can be varied by the use of the Maddox rod or 
the Maddox groove, either of which, placed before the eye, 
elongates the flame into a long, narrow streak of light, the 
position of which, in relation to the normal flame, as seen by 
the other eye, will indicate the presence, or absence, of any 
muscular insufficiency. If the streak of light is directly over 
the flame, the muscles are normally balanced—a condition of 
orthophoria. If the line of light is to one side or the other of 
the flame, muscular insufficiency is proven—a condition of 
heterophoria. 

The Maddox rod or groove is placed before the eye in the 
horizontal position when it is desired to test the internal and 
external recti muscles, and in the vertical position when the 
superior and inferior recti muscles are under examination, 
which means that the line of light should be at right angles to' 
the deviation which it is desired to measure. The degree of 
the heterophoria and its nature will be determined by the 
strength of the prism and the position of its base, which are 
required to bring the line of light directly over the candle 
flame. 

COMPOUND MUSCULAR INSUFFICIENCIES. 

In addition to the tests for determining a weakness of any 
of the recti muscles, we have a test for detecting compound 
muscular defects (that is, of both the vertical and horizontal 
muscles), and, also, for examining the oblique muscles. This 
test is made by a double prism, which consists of two prisms 
set in a rim, with their bases together. 

In using this test with both eyes open, three images are 
seen, and the position of the middle image will indicate the 


294 


METHOD OF EXAMINATION. 


presence or absence of any muscular insufficiency, and what 
muscles are affected, if any. In this way can be detected an 
insufficiency of any one of the recti muscles, or of either of the 
oblique muscles. 

In examining the acuteness of vision and determining the 
refraction of the eye, the card of test-letters should be so hung 
that the best possible light may fall upon it. 

In determining the refraction of an eye, the test should 
always commence with convex lenses. 

Each eye should be tested separately, commencing usually 
with the right eye, or with that eye that possesses the greater 
acuteness of vision, while the other eye is excluded from 
vision by an opaque metal disk placed in the trial-frame. 

After having ascertained the refraction of each eye 
separately, the two eyes are then to be tried together; and it 
will be found that binocular vision is better than that of either 
eye separately; with the two eyes, stronger convex glasses 
will be accepted in hypermetropia, and weaker ones may be 
required in myopia, than the test of a single eye would 
indicate. 


CHAPTER X. 


PRESBYOPIA. 

Before entering upon the consideration of the subject of 
presbyopia, it may be well for the student that a little atten¬ 
tion be given to the accommodation of the eye. As is the case 
with all optical instruments, so the dioptric apparatus of the 
eye, when in a state of repose or when not changed by mus¬ 
cular effort, can receive distinct images of objects only when 
they are situated at one and the same distance. When the 
distance is changed the apparatus must be modified, or the dis¬ 
tinctness of the image will be marred. 

In a condition of perfect repose the eye exercises its mini¬ 
mum power of refraction, and is then adapted to the greatest 
distance at which it is able to see, that is, its far point, and 
cannot then see distinctly those objects which are close at 
hand. In this condition of inaction of the accommodation, the 
emmetropic eye is adapted for infinite distance, or for the paral¬ 
lel rays which proceed from, and represent, infinity; the hyper¬ 
metropic eye is adapted for a point beyond infinite distance, 
that is to say, for rays converging toward its far point situated 
at a distance; the myopic eye, whose far point is situated at a 
certain fixed distance in front of it, is adapted for that dis¬ 
tance, or for the diverging rays proceeding from that distance. 
In each of these cases the parallel rays, the converging rays 
and the diverging rays are respectively united on the retina, 
and form a perfect image there without any effort of the 
accommodation. 

An eye, when in a condition of repose, as above, and 
adjusted for its far point, cannot distinctly see objects at any 
nearer distance, because its dioptric apparatus is too feeble to 
bring the more diverging rays proceeding from nearer objects 
to a focus on the retina. An example may be taken of an em¬ 
metropic eye: Its far point being situated at infinity, the eye is 
adapted for the parallel rays that proceed from that distance, 
and rays from no other point are focused on the retina. 

295 



296 


PRESBYOPIA. 



In the above illustrative figure, parallel rays are united in 
an exact focus on the retina, while the divergent rays proceed¬ 
ing from a near point, as at A, are united behind the retina at 
the point B. In order that these divergent rays may be united 
on the retina, it is necessary either to render them parallel, or 
to increase the refracting power of the eye to such a degree 
that it will unite them on the retina instead of at B. 

If there is placed in front of the eye a convex lens of such 
strength that its focus would be at A , it would render parallel 
the rays coming from A , and make them the same as if they 
came from infinite distance. The emmetropic eye, by the aid 
of this glass, will therefore see as well at the short distance A 
as it does at infinite distance without any glass, in both cases 
with the eye at rest, and without any effort of accommodation. 

In this way the focus of the eye could be changed and its 
vision adapted for objects at different distances, but outside of 
this the fact remains that the eye is able to see close at hand as 
well as at a distance, and that, too, without the intervention of 
any convex lens. It is necessary, however, that a certain very 
short time elapse in passing from the fixation of an object at a 
distance to one near at hand, and with this change there is 
oftentimes a consciousness that the eye makes a certain effort 
in altering its point of fixation. In making this change, the 
effort exerted has added to the dioptric system of the eye the 
convexity necessary to enable it to see objects close at hand, 
This increase in the refracting power necessary to change the 
adaptation of the eye from a far point to a near point, an in¬ 
crease which the convex lens accomplished when the eye was 
in a condition of repose, is now effected in the eye itself. This 
is accomplished by the crystalline lens, which undergoes a 
change of form necessary to accommodate the eye for objects 
close at hand, which change consists in an increase in its con¬ 
vexity, and, therefore, in its refractive power. 








/ 


PRESBYOPIA. 297 

The function of accommodation, or the change in the ad¬ 
justment of the eye for vision at different distances, is effected 
by means of the action of the ciliary muscle, which, by its con¬ 
traction, causes the ciliary body to advance. The ciliary liga¬ 
ment is thereby relaxed, and the lens, which had been more or 
less flattened by its tension, is left to its own inherent elasticity 
and assumes a more convex shape anteriorly, while the poste¬ 
rior surface, incased in the vitreous humor, preserves its form 
almost unaltered. 

The closest point for which the eye can accommodate 
itself is called the near point. The distance between the far 
point and the near point is called the range of accommodation. 
It is the distance over which the eye has command by the aid 
of its accommodation. The force necessary to change the eye 
in its adaptation from its far point to its near point is called 
the amplitude of accommodation. Consequently, the ampli¬ 
tude of accommodation is necessarily represented by the dif¬ 
ference in the refractive power of the eye when in a state of 
complete rest, and when at its maximum of accommodation. 

Now, since the accommodation has the same effect as a 
convex lens which would enable the eye, when deprived of its 
accommodation, to see at its near point, we are able to express 
the power of the accommodation by the number of this lens. 
The accommodation is therefore equal to a convex lens of 
such strength as would give to rays, proceeding from the near 
point, a direction as if they came from the far point. Now the 
question occurs, What will be the strength of such lens? 

As has already been stated, the focus of the lens of the 
emmetropic eye should coincide with the near point, since it 
should render parallel the divergent rays proceeding from that 
point. Its focal distance is, therefore, equal to the distance 
which separates the near point from the eye. If this distance 
is ten inches, the lens will have a refracting power of 4 D., and 
the amplitude of accommodation will equal 4 D. 

The whole of this refracting power serves to adapt the eye 
for positive points situated within infinite distance. In order 
to determine the amplitude of accommodation of an emme¬ 
tropic eye, we need only find the shortest distance at which the 
person can read the smallest printed characters. This distance 


298 


PRESBYOPIA. 


is the focal distance of the lens corresponding to the amplitude 
of accommodation. 

As has already been stated, the accommodation depends, 
on the one hand, on contraction of the ciliary muscle, and on 
the other hand, on the elasticity of the lens. As age advances, 
the ciliary muscle loses, by degrees, its power of contractility, 
and in like manner the lens loses its elasticity. These two 
factors, the loss of strength of the ciliary muscle and the in¬ 
creasing hardness of the lens, have, necessarily, a restricting 
influence upon the accommodative power. 

When we come to think of it, it does seem somewhat 
strange that this diminution of the accommodative power does 
not wait for the usual decrepitude that accompanies and con¬ 
stitutes old age, but that it begins at a time when all the other 
faculties are progressing in their development. Already, at 
the tenth year, the accommodative power begins to weaken 
and its amplitude to diminish. 

Bonders, who discovered this fact and established the 
laws that govern it, has prepared a diagram which represents 
the amplitude of accommodation at the different periods of 



The figures on the horizontal line of the diagram indicate 
the ages, and those on the vertical line, to the left, the corre- 





































PRESBYOPIA. 


299 


spending dioptres of accommodation. The curve a b corre¬ 
sponds to the refraction of the eye in a condition of repose, or, 
in other words, when at its minimum of refraction; that is to 
say, to the refracting power which the eye represents when 
adapted for its far point. 

As will be seen in the diagram, this does not change up to 
the age of fifty years, but from that time on it diminishes; 
the emmetrope becomes hypermetropic; the hypermetrope 
more hypermetropic; and the myope loses a part of his my¬ 
opia, and may become, according to its degree, emmetropic, 
or even hypermetropic. 

The curve c d indicates the maximum of refraction of 
which the eye is capable, that is to say, the sum of the refract¬ 
ing power which the eye represents in a state of repose, and 
what it is able to add to itself by putting into play all its 
power of accommodation, or, expressed still differently, the 
refracting power which the eye possesses when it is adapted 
to its near point. 

As can be seen by the diagram, the positive refracting 
power of the eye diminishes gradually, and becomes, from the 
age of sixty-five, feebler than the minimum of refraction was 
in the preceding years. In spite of this, however, there still 
remains some accommodative power so long as the two curves 
do not meet, because the passive refraction of the eye also 
diminishes from the fifty-fifth year. It only ceases at the age 
of seventy-three years, when the two curves meet. 

The amplitude of accommodation is represented for each 
age by the number of dioptres comprised between the two 
curves, on the vertical line corresponding to the age, which 
means that the amplitude of accommodation is represented by 
the difference in the refraction of the eye when in a state of 
absolute rest and with its accommodation completely relaxed, 
and when the eye is exerting its entire force and its accommo¬ 
dation is at the greatest tension. By following these lines in 
working out the problem, there has been prepared the follow¬ 
ing 


300 


PRESBYOPIA. 


table: 


Amplitude of 


Age. 

accommodation. 

io Years. 

14 D. 

15 “ 

12 “ 

20 “ 

10 “ 

25 “ 

8.5 “ 

30 “ 

7 “ 

35 “ 

5-5 “ 

40 “ 

4-5 “ 


Age. 

Amplitude of 
accommodation, 

45 Years. 

3-5 D- 

50 “ 

2.5 “ 

55 “ 

L 75 “ 

60 “ 

1 

65 “ 

o .75 “ 

70 “ 

0 25 “ 

75 “ 

o.co “ 


The amplitude of accommodation is just the same in an 
eye with an error of refraction as in a normal eye, and, there¬ 
fore, the figures in the table apply equally to all the errors of 
refraction to which the eye is subject, and to all degrees of the 
various errors. The amplitude of accommodation is the same 
in all these defects, but the positive refracting power of the eye 
is not the same, but it is composed of, or equal to the sum of, 
the refraction which the eye represents when in a condition of 
repose and with its accommodation at rest, and that which can 
be added by the strongest effort of its accommodation. 

In emmetropia, and in emmetropia only, is the positive 
refracting power of the eye (that is, the strongest refracting 
power which the eye is able to assume by the greatest effort of 
its accommodation which adapts it to its near point), identical 
with, and equal to, amplitude of accommodation. 

In hypermetropia there is no such a thing as perfect 
repose for the eye, nor can the accommodation of the hyper¬ 
metropic eye ever be at rest, because clear vision, even at a 
distance, is obtained only by an effort of the accommodation. 
The far point of such an eye is situated beyond infinity, and, 
consequently, it is a negative quantity. Here a portion of 
the amplitude of accommodation must be used to correct the 
hypermetropia and to adapt the eye for infinity, and, therefore, 
the positive refracting power of a hypermetropic eye is equal 
to the amplitude of its accommodation, diminished by the por¬ 
tion that is necessary to adapt it for infinity. 

In myopia, on the other hand, the accommodation is but 
little needed, because the eye is adapted for its far point, which 
is situated at a certain definite distance; and, when adjusted for 
this far point, the accommodation is at rest. The far point of 







PRESBYOPIA. 


301 

such an eye being situated at a finite distance, its condition 
of repose is positive, or represents a quantity of positive refrac¬ 
tion. Here, then, a portion of the positive refracting power of 
the eye is supplied by its optical condition, and augments its 
amplitude of accommodation by that amount (instead of sub¬ 
tracting from it, as in hypermetropia), and, therefore, the 
positive refracting power of a myopic eye, that is, its total 
amount, is equal to the sum of the amplitude of its accommo¬ 
dation and that power of refraction which is represented by the 
eye in a state of repose. 

Suppose we represent the positive refracting power of the 
eye by the abbreviation pas. ref., and the amplitude of accom¬ 
modation by amp. acc., and the condition of repose, when the 
accommodation is at rest, by rep. From this we prepare the 
following 

table: 

Emmetropia. Pos. ref. = amp. acc. 

Hypermetropia. Pos. ref. = amp . acc. less rep. 

Myopia. Pos. ref. = amp. acc. plus rep. 

The distance of the near point from the eye is equal to the 
focal distance representing the total amount of the positive 
refracting power (pos. ref.). In view of this fact, it follows that 
the near point is not always situated at the same distance in 
the different states of refraction. Even when the amplitude of 
accommodation is the same the near point will always be 
farther removed in hypermetropia, and brought closer in my¬ 
opia, than in emmetropia. 

As an example, we will take an emmetropic person twenty 
years of age. A reference to the table shows that he possesses 
10 D. of accommodation, and, therefore, the distance of his 
near point from the eye will be to centimeters ( yy = 10), or 
four inches. 

A hypermetropic person of 4 D., at the age of twenty, 
will also possess an amplitude of accommodation of 10 D., 
but his near point will not be at four inches, as in the case of 
the emmetrope, but will be further removed, according to the 
following formula: 

Pos. ref. = amp. acc. less rep. 

Pos. ref. = 10 D. — 4 D = 6 D. 


302 


PRESBYOPIA. 


This makes the distance of 'the near point in a hyperme- 
trope of 4 D. at 16 centimeters (^f- = 16), or six inches. 

On the other hand a myope of 4 D. also possesses, at the 
age of twenty, an amplitude of accommodation of 10 D., but 
his near point will not be at four inches, as in the emmetrope, 
nor at six inches, as in the hypermetrope, but will be much 
nearer than in either case, according to the following formula: 

Pos. ref. — amp. acc. plus rep. 

Pos. ref. = 10 D. -f- 4 D. = 14 D. 


This places the distance of the near point in a myope of 4 D. 
ait 7 centimeters ( Yr ~ 7)> or a little less than three inches. 

Therefore, the distance of the near point in emmetropic 
persons corresponds to the focal distance of the lens which 
represents its amplitude of accommodation, according to the 
following 

table: 



Amplitude 

Distance 


Years. 

of accommodation. 

of near point. 

Inches. 

IO 

14 D. 

7 cm. 

3 

15 

12 

8 “ 

3i 

20 

10 “ 

10 “ 

4 

25 

8.5 “ 

12 “ 

5 

30 

7 “ 

14 “ 

5l 

35 

55 “ 

18 “ 

7 

40 

4-5 “ 

22 “ 

9 

45 

3-5 “ 

28 “ 

11 

50 

2.5 “ 

40.5 “ 

16 

55 

1-75 “ 

57 “ 

23 

60 

1 “ 

100 “ 

40 


In order to determine the distance of the near point in 
patients with defective vision, we ascertain the maximum 
power of refraction which the eye possesses (pos. ref.), and the 
focal distance of the lens which represents this will be the dis¬ 
tance of the near point. In order to determine the positive 
refracting power in hypermetropia, we must subtract from the 
pos. ref. of emmetropia the number of dioptres which repre¬ 
sents the degree of the hypermetropia, while in myopia we 
must add to the pos. ref. the number of dioptres which repre¬ 
sents the degree of myopia. 




PRESBYOPIA. 


303 


The recession of the near point is so constant and regular 
that some authorities think they are able to determine from it, 
with considerable precision, the age of the individual by taking 
into account the state of refraction of the eye. 

The near point of distinct vision, which, in childhood, is 
very close, gradually recedes with each year of life, this reces¬ 
sion commencing as early as the tenth year, and steadily and 
constantly progressing, as can be seen bv reference to the 
above table. 

In regard to this table, it may be said that the figures do 
not invariably represent the total amount of accommodation it 
is possible for the eye to make by the strongest effort it can 
exert. For instance, at the age of forty-five, it is stated that 
the nearest point of clear and comfortable vision is at eleven 
inches, but in such a case it may be found that the eye, by an 
extraordinary effort of its accommodation, will be able to read 
for a very short moment at a closer point—*say at, perhaps, ten 
inches. 

As the near point gradually and steadily recedes farther 
and farther from the eye, it must finally pass beyond the dis¬ 
tance at which we are accustomed to read and write and use 
our eyes for the ordinary occupations of life. When the near 
point has passed this limit, it is evident we will begin to be re¬ 
stricted in our work. Even at a time when the total effort of 
•our accommodation yet suffices to keep the near point at the 
customary position, work at that distance must soon become 
very fatiguing, and even impossible, because it is accomplished 
only by the aid of the maximum contraction of the ciliary 
muscle. 

The condition of the eye when the near point has passed 
beyond the usual distance for reading and writing and work, is 
a matter of considerable importance; and from the fact that 
this diminution in the power of accommodation does not cause 
any inconvenience until middle age is reached, and near vision 
continues satisfactory up to that time, the name presbyopia has 
been given to this condition, which word is made up of two 
Greek words meaning old vision. It means that the ciliary 
muscle is no longer equal to the task of affording clear vision 
at near distances. 


304 


PRESBYOPIA. 


For forty years and more this little muscle of accommoda¬ 
tion has been brought into constant use during our waking 
hours, and has been an indispensable companion in all our 
occupations. Only by its use have we been able to read the 
daily news and keep ourselves informed of occurrences of in¬ 
terest at home and abroad. Only by its assistance have we 
been able to gain an education and store our minds with useful 
knowledge, or while away some tedious hour by a perusal of 
the latest novel. Only by its aid are civilization and the spread 
of intelligence possible; and without its beneficent offices we 
would be relegated to the condition of the untutored savage. 

But, finally, this prop on which we have been accustomed 
to lean for so many years begins to weaken, and seems loath to 
still perform its function with its old-time fidelity, and we are 
forced to realize that we can no longer depend on it for near 
vision in our ordinary occupations of life, and we are then 
compelled to look elsewhere for that assistance which Nature 
is beginning to refuse us. 

In searching for the time when glasses were first used for 
the improvement of vision, we are carried back to the thir¬ 
teenth century, when we find they were employed to assist the 
vision of old people for work close at hand, or for the relief of 
the condition which we now call presbyopia. It is said that 
Roger Bacon, who was the professor of philosophy in the 
University of Oxford, indicated how much benefit old men 
might derive from looking at letters through a convex lens, 
and mentioned the magnifying properties of such lenses. 

The use of the convex lens as a magnifying glass, and the 
making of binocular spectacles for use in overcoming the defi¬ 
ciencies of the eyes induced by age, imply a great stride in the 
development of the optician’s art. In comparison with the 
elegant and finely-finished spectacles and eye-glasses that are 
produced by the manufacturing optician of the present day, 
those clumsy, ill-shaped spectacles of the olden time seem very 
crude indeed, and we are almost disposed to sneer at them; 
and yet we cannot doubt that they were of the greatest value 
to the studious men of that day, and, indeed, we cannot help 
but wonder how people got along without them previous to 
that time. As night now is made almost as bright as day by 


PRESBYOPIA. 


305 


the glare of electric lights of numberless candle-power, the 
same wonder arises how our forefathers lived and progressed 
and accomplished their wonderful strides by the aid of pine 
knots and tallow dips. But, then, books and magazines and 
newspapers were not a part of the daily life of that day, and, 
therefore, the same necessity for the constant use of the eyes 
for letters did not exist, nor was the necessity for artificial aid 
to the eye felt as at the present time. 

Now they are a necessity and a comfort to the aged mem¬ 
bers of every household; “for it is not too much to say that 
through the aid of spectacles we continue in the enjoyment, 
even in old age, of one of the most noble and valuable of our 
senses. They enable the mechanic to continue his labors, and 
the artist to display his skill, in the evening of life; the scholar 
pursues his studies by their help, adding to the knowledge of 
others, and recreating his own mind with intellectual pleasures, 
thus passing days and years in satisfactory occupation that 
might otherwise have been devoured by melancholy, or wasted 
in profitless idleness.” 

“This return to juvenility of sight is one of the most 
agreeable experiences of middle age. It cannot be too gener¬ 
ally understood that spectacles, instead of being a nuisance or 
an incumbrance, or an evidence of bad sight, are, to the pres- 
byope, a luxury beyond description, clearing outlines which 
were beginning to be shadowy, brightening colors which 
were beginning to fade, and instantly restoring near vision to 
a point from which, for ten or a dozen years previously, it had 
been slowly and imperceptibly, but steadily, declining.” 

Many persons strive to fight against the approach of pres¬ 
byopia, and vainly imagine they can do something by their 
own effort to retard or delay its coming. But it is a vain and 
foolish struggle against the inevitable. Just as sure as the sun 
will rise to-morrow morning and set to-morrow evening, just 
so sure will the eye begin to fail, for reading vision, as the 
person reaches middle life. It is not only useless, but posi¬ 
tively harmful, for any man to fight against Nature in regard 
to the changes it effects in the human eye, and the sooner 
people can be convinced of this fact, and can be induced to 
accept the assistance which art offers in place of waning 


3°6 


PRESBYOPIA. 


natural powers, the better for them. They may resort to im¬ 
prudent artificial means to neutralize the approach of age, and, 
in some ways, may succeed in deceiving others, and even 
themselves. But, as regards failing eye-sight for reading, it 
cannot safely be disregarded, and any effort to deceive others 
will result in more badly deceiving themselves. 

When the optician speaks of these matters, and urges his 
friends and customers to accept the inevitable, and tries to con¬ 
vince them of the absolute necessity of commencing to wear 
glasses just as the first symptoms of presbyopia manifest them¬ 
selves, it may, at first sight, seem as if he was talking selfishly, 
and from self-interest, and solely for his own benefit. But, as 
the public becomes better educated in these matters, they will 
begin to realize that the benefit is on their side just as much as, 
and more so than, on the optician’s, and they will be forced to 
admit that the spectacle man knew what he was talking about; 
and, until that time, the optician must be willing to suffer mis¬ 
apprehension, content with the knowledge and consciousness 
of his own rectitude in the matter. Every day isome individual 
learns, from -sad experience, and, perhaps, when too late, that 
he has trifled with and neglected his sight, until his eyes have 
been permanently and seriously injured. 

It is possible for the use of glasses to be deferred for quite 
a long time after they are really needed; but the delay is ac¬ 
complished only at great cost, and has to be paid for in some 
way, sooner or later, and with the addition of the highest rate 
of compound interest. 

An illustration may be used of a costly engine, finely 
finished and nicely adapted in all its parts; constant oiling 
keeps this delicate piece of mechanism running smoothly and 
without the slightest evidence of friction, and, apparently, 
without any wear and tear or injury to any of its parts. Sup¬ 
pose the oiling was neglected for a day or two; the engine 
would not refuse to run, but would continue to perform its 
functions, but with considerable more of an effort. The oiling 
may still be neglected and the engine still continue to run and 
accomplish its work, but at the expense of so much effort and 
so much friction that the finely-balanced piece of mechanism 
is soon irreparably damaged, and rendered worthless. 


PRESBYOPIA. 


307 


CHANGES THAT TAKE PLACE IN THE EYE WITH THE ADVANCE 

OF AGE. 

The acuteness of vision for distant objects must vary, 
more or less, with the degree of transparency of the atmos¬ 
phere. So, also, the distinctness of the retinal images must 
vary, more or less, with the degree of transparency of the re¬ 
fractive media of the eye. This transparency of the humors of 
the eye is most perfect in childhood, a fact which can be ascer¬ 
tained by the use of the ophthalmoscope, and it gradually and 
.slightly diminishes with the advance of age. Therefore it 
follows, as a natural sequence, that the acuteness of vision 
must be greater in childhood than at any other period of life. 

The diminution in the transparency of the refractive 
media of the eye is evidenced by a lack of the natural lustre of 
the cornea, and the formation of the arcus senilis near its 
margin; by the formation of folds of the membranes of the 
vitreous and by the increase of the number and size of the 
muscse volitantes; the layers of the crystalline lens become 
turbid and its nucleus assumes a yellowish tint, and the retina, 
also, becomes -slightly opaque. An authority says “The dimi¬ 
nution of transparency of the refractive media progresses with 
such uniform regularity with advancing years, that practiced 
ophthalmoscopists are able to approximate the age of the 
patient by observing the clearness with which the fundus of 
the eye can be seen.” This is a strong statement, and one that 
almost staggers belief; but the possibility of such a thing 
serves to prove, not only that there is a loss in the transparency 
of the media, but that this loss is a matter of steady and 
gradual growth. 

Some changes may also occur in the sclerotic, choroid 
and iris; the pupil, also, gradually lessens in size, and thus 
diminishes the quantity of light admitted into the eye, which 
decrease of light must be compensated for by an increase in 
the degree of illumination. Senile changes also take place in 
the optic nerve, whereby its conductive power is diminished 
and its perceptive power also blurred; similar changes occur¬ 
ring in the retina. 

The changes which occur in the eye all tend to cause a 


3°8 


PRESBYOPIA. 


decrease in its refractive power and a lessening of its range of 
accommodation. The crystalline lens becomes firmer and 
harder, and, as the lines of separation between its different 
layers become less distinctly marked, the lens appears to be¬ 
come more homogeneous, and also somewhat flatter, and, as- 
a consequence, it loses part of its refractive power. The in¬ 
crease in the firmness of the lens makes it more difficult to be 
acted on by the ciliary muscle in the production of the degree 
of curvature of its surfaces necessary for the distinct vision 
of small objects close at hand, and in this way diminishes the 
range of accommodation. 

The gradual diminution in the refractive power of the eye 
proceeds until the time arrives, at middle age or later, when its- 
condition has changed to one of acquired hypermetropia. 
Many of these changes in the eye commence in childhood, but 
their progress is so slow that they escape notice. The defects* 
can, in earlier years, in a great measure, be partly neutralized 
by an increase in the illumination and by holding small ob¬ 
jects farther from the eyes, until, finally, they become suffi¬ 
ciently manifest to interfere with the function of vision for 
objects close at hand, and the unassisted eye is no longer able 
to perform its functions in the ordinary occupations of life. 

CHANGES IN THE LENS. 

While the changes that take place in the eye with the- 
advance of age are many, yet the principal cause of the dimin¬ 
ished refraction of the eye and of the approach of presbyopia 
must be sought for in the changes that take place in the crys¬ 
talline lens. 

There was at one time an erroneous theory prevalent that 
the cause of the diminution of the accommodative power was- 
due to a flattening of the cornea, and this opinion was believed 
and taught by all the older physiologists, until the investiga¬ 
tions of modern science proved its fallacy, and it may even yet 
be found in some of the popular treatises on optics, which are 
used in elementary teaching. But the measurements that have 
been made of the convexity of the cornea seem to prove the 
utter falsity of this teaching, and that if there is any change in 
the curvature of the cornea, it is in the direction of becoming- 


PRESBYOPIA. 


309 


slightly more convex. Hence, some other cause must be 
sought for to explain and account for the changes that take 
place in all eyes, and which, even in childhood, cause the near 
point to gradually recede, until, finally, after middle age, em- 
metropia gives way to acquired hypermetropia; and this cause 
is found in the changes which the crystalline lens undergoes. 

THE CRYSTALLINE LENS. 

The crystalline lens is a lens-shaped body (as its name 
would indicate), and in health is perfectly transparent. Its 
anterior and posterior surfaces are both convex, or, in other 
words, it is a biconvex lens; but the curvatures of both sur¬ 
faces are not equal, the posterior surface having a much 
stronger curvature than the anterior. The lens lies in a de¬ 
pression in the vitreous humor made to receive it, and is 
directly behind the pupil and in contact with the pupillary 
edge of the iris. 

The crystalline lens consists of a transparent capsule and 
the lens substance proper. The curvature changes with age, 
it being more convex in childhood, and becoming gradually 
flatter with the advance of age. Its consistency is much 
firmer than that of the vitreous body. Its transparency is 
gradually changed to a yellowish-amber tint with advancing 
years. 

In childhood the lens is soft, and quickly yields to the 
action of the ciliary muscle, changing its form and convexity 
with the greatest ease. The nucleus of the lens is somewhat 
firm, but the balance of the structure is soft and pliable, the 
softness and elasticity increasing more and more toward the 
periphery. The refractive power of the crystalline lens is at 
its greatest in childhood, at which time, on account of its soft¬ 
ness and elasticity, which are particularly noticeable in its 
outer layers and peripheral portions, its convexity is easily 
increased by the action of the ciliary muscle to the highest pos¬ 
sible degree. 

As years pass by and age creeps on, the outer layers of 
the lens gradually increase in firmness until they approach the 
consistency of the nucleus, while ttye lines of separation be¬ 
tween the different layers become less and less marked. 


3io 


PRESBYOPIA. 


making the lens more homogeneous in structure, both of 
which causes act to diminish its refractive power. Its in¬ 
creased firmness renders it less yielding to the action of the 
muscle of accommodation; consequently, when the usual de¬ 
gree of muscular force is applied, it produces much less effect 
on the firmer and more uniformly homogeneous crystalline 
structure than on the softer laminated lens of youth. In addi¬ 
tion to the inability of the ciliary muscle to produce the same 
degree of convexity of the lens as in childhood, there is also a 
diminution in its actual refractive power. 

Senile changes take place in all eyes, the same in myopic 
and hypermetropic as in emmetropic eyes; but, of course, the 
changes are more marked and more noticeable in hyperme¬ 
tropic eyes, and less manifest in myopic eyes. At this time the 
myope has the greatest advantage, because in myopia there is 
an excess of refractive power in the eye, and the far point of 
distinct vision is uncomfortably close. The presbyopic 
changes, in diminishing the refractive power of the eye and in 
removing the reading point to a more convenient distance, 
tend only to place the eye in a more normal condition for 
satisfactory and comfortable use. 

DONDERS’ OPINION. 

It is for these reasons that Donders gives his preference to 
this myopic condition of refraction, and as he has been one of 
the masters in the science of optics, and as his great work on 
“Refraction and Accommodation” has been the source from 
wnich much of our knowledge has been drawn, we will give 
his words on this subject verbatim: 

“Finally, should the question be proposed whether emrne- 
tropia is the most desirable condition: As concerns myself, I 
should give the preference to a slight degree of myopia, and I 
shall subsequently state my reasons for doing so. 

“The inconvenience to youth from a slight indistinctness 
of more remote objects is more than compensated for by the 
improved vision of middle and advanced life. Herein the 
myope finds a compensation for what he loses with reference 
to the vision of remote objects. The advantage is not small. 
Up to the sixtieth, or even the seventieth, year of our age, not 


PRESBYOPIA. 


311 

to need spectacles in order to see accurately whatever comes 
immediately under our eyes is a great privilege. 

“This privilege belongs to a myopia of from to A(3 D. 
to 4 D.), in which the eye is not threatened with any special 
dangers. With slighter degrees of myopia a good deal of this 
privilege is still enjoyed. This is a condition which may well 
be envied by emmetropic eyes. I never found a normal eye 
which participated in the same advantage. Many persons, 
however, suppose they are so highly privileged. Almost daily 
it occurs that at fifty-five years of age the distance of the far 
point lies at only from eight to ten inches, and spectacles are 
not thought of. Such people consider themselves a lucky ex¬ 
ception. They are extremely proud of their sharp sight. The 
inquiry whether they are near-sighted is answered in the nega¬ 
tive, with a smile of self-complacency. 

“At a distance of twenty feet hang Snellen’s letter^tests; 
lines 20 and 30 they do not recognize; 40 not at all or scarcely; 
50 and 60 are the first which are easily recognizable to them. 
Not until they try glasses of — or — do they well dis¬ 
tinguish line 20, or at least 30, with accurate contours. Re¬ 
luctantly they acknowledge themselves beaten.” 

There are many persons met with in our daily experience 
who are able to read fine print, without convex glasses, at fifty 
or sixty years of age, and who regard it as an evidence that 
their eyes have escaped the customary senile changes. To 
such persons the above extract from Donders would be inter¬ 
esting reading and a revelation. They were myopic in youth, 
although they were probably unaware of the fact. Persons 
frequently discover that their eyes are near-sighted by acci¬ 
dentally trying on a pair of concave spectacles, and they are 
surprised to find that distant vision is greatly improved, so 
much so that they are often impelled to ask the question if 
other people can see as clearly at a distance as they do after 
having placed before their eyes a pair of concave glasses, such 
a bright and new world having been opened up to them. 

It often happens in the experience of many an optician, 
while ascertaining the acuteness of vision of a patient sitting in 
the chair and looking at the test-card hanging twenty feet 
away, that an accompanying friend expresses astonishment 


312 


PRESBYOPIA. 


that the patient should be able to read the No. 20 line at 
twenty feet. He finds he is unable to do it, and yet, if you ask 
him if he is near-sighted, he .says no, he can see as far as any 
one. When he is placed in the chair, after the patient has 
vacated it, and he is given concave glasses of 1 D. or 2 D., he 
is at once enabled to read this line with the greatest ease,and he 
learns, for the first time, that his eyes are myopic. At fifty-five 
or sixty years of age he will be able to read and write and per¬ 
form all the ordinary occupations of life without the assistance 
of convex glasses, and if he had not accidentally learned that 
his eyes were myopic, he would naturally have thought that 
they had escaped the usual senile changes. 

PRESBYOPIA NOT ABNORMAL. 

Presbyopia is not an optical defect—it is a physiological 
change. It should not be considered a disease, or a defect, or 
a departure from a normal condition. It is simply the result, 
and an invariable accompaniment, of old age, just as gray 
hairs, and other evidences of change and decay, make their 
appearance at the usual time, to show that we are gradually 
advancing in years. Presbyopia should, therefore, be looked 
upon as a natural condition of the eye, or a natural change in 
the condition of the eye, which is common to every person, 
and from which none can escape, except as it may be influ¬ 
enced by the refractive condition of each particular eye. 

The state of the general health will frequently have a de¬ 
cided influence on the appearance of presbyopia, as a weakness 
of the ciliary muscle is often found in cases of general bodily 
weakness. 

Presbyopia, in every case, will be modified by the refrac¬ 
tion of the patient’s eye. Hypermetropia increases it, in¬ 
tensifies it, and causes it to appear much earlier than other¬ 
wise; in fact, hypermetropia often exists without any special 
symptoms, and without causing any special discomfort, and 
only makes itself manifest as an early presbyopia. Hyperme¬ 
tropia causes a deficiency in the refractive power of the eye 
from the start, and as presbyopia also produces a gradual loss 
of the same refractive power with the advance of age, both 
conditions act in the same direction, and pull together, as it 


PRESBYOPIA. 


31.3 


were, in producing the symptoms of presbyopia, which, in 
such a case, are augmented. 

In myopia, on the other hand, we have a condition of re¬ 
fraction which tends to diminish and neutralize presbyopia, 
and to delay its oncoming until much later in life; and, in fact, 
when the myopia is of sufficient degree (— 4 D., or more), 
presbyopia may never occur, because, in such a case, the near 
point can never recede to an inconvenient distance. Myopia 
carries with it an increase of refractive power from the start, 
and, as presbyopia causes a gradual loss of refractive power, 
these two conditions pull in opposite directions, neutralizing 
each other and retarding the approach of presbyopia. 

For these reasons it follows that no rules or tables can be 
prepared that will be of any very great practical value in the 
correction of presbyopia. If every eye possessed the same de¬ 
gree of refractive and accommodative power, and if every eye 
was equally affected by the inevitable senile changes, then 
every eye at a certain given age, the same for the whole 
human race, would begin to be presbyopic and would require 
the same glasses, in which case every age would call for a cer¬ 
tain number of lens, and every year of life would indicate a 
certain definite increase in the strength of the glasses. Under 
such circumstances, the fitting of glasses for the correction of 
presbyopia would be ais easy as rolling off a log, and would 
require no special optical skill or knowledge; in fact, the 
glasses could be selected by the patient himself, and without 
any assistance. 

But such is not the case, and the only proper rule that can 
be laid down for the guidance of the optician in the manage¬ 
ment of the cases of presbyopia that come to him for glasses, is 
to test each case carefully on its own merits, and determine the 
condition of the refraction for each person separately, and use 
this as the starting point from which to determine the glasses 
required, and without any reference to any ready-made tables 
on the subject. 

WHEN DOES PRESBYOPIA COMMENCE? 

This question carries with it the thought that presbyopia 
has a commencement, that it is not congenital, that it does not 


3i4 


PRESBYOPIA. 


exist indiscriminately at any age of life, that it does not make 
its appearance at any irregular time, or from any indefinite 
cause, but that it is a defect or a change that is due to definite 
and natural causes, and that it does not come on unexpectedly 
or without warning; but that, sooner or later, with the excep¬ 
tions that will be mentioned, it comes to every individual 
whose eyes are used for close or near vision of any kind. 

Such being the case, the question very naturally occurs,. 
At what age does presbyopia commence? As has already been 
stated, presbyopia comes on with the advance of years, and 
usually makes its appearance at what is termed middle age. 
Optical students, pursuing a course of instruction, are fre¬ 
quently asked this question, and the usual answer is, “At the 
age of forty-five years.” 

Now, is this answer correct? It cannot be said to be abso¬ 
lutely incorrect; and yet, at the same time, it is not a complete 
and proper answer. It should be remembered that a gradual 
recession of the near point is one of the inevitable accompani¬ 
ments of advancing years, and it is on this recession of the near 
point and its location that the commencement of presbyopia 
depends. The near point begins to recede very early in life 
(about the tenth year); but at this early age, and for some time 
thereafter, it has noit receded to any inconveniently great dis¬ 
tance, but is still near enough to the eyes as not to make any 
difference in their comfortable use, nor is reading or writing 
accompanied by any symptoms of asthenopia; and, hence, 
presbyopia does not commence until this near point has re¬ 
ceded to such a distance as to make near vision (reading, 
writing and close work) inconvenient or impossible. 

Then, in place of the question, At what age does pres¬ 
byopia commence? the following query may be substituted: 
How far may the near point recede before painful and un¬ 
pleasant symptoms are experienced in reading, or what is the 
farthest near point beyond which the symptoms of presbyopia 
become apparent, and the need of glasses, or of some artificial 
assistance, is experienced? 

The answer to these questions has been determined more 
from practical experience, and the actual facts gained thereby, 
than from any nice scientific discovery or complicated mathe- 


PRESBYOPIA. 


315 


matical calculation. It has been found that when the near 
point has receded beyond eight or nine inches, then near vision 
cannot be kept up with the same ease and comfort as before, 
and there is an uncontrollable desire to remove the work or 
paper farther from the eyes. The point of departure, then, we 
shall fix at eight inches, and the commencement of presbyopia 
in each case will depend on the near point having reached or 
passed this limit. 

DEFINITION OF PRESBYOPIA. 

Presbyopia, then, may be defined as that condition of the 
eye in which the near point has receded beyond this limit of 
eight inches, and the person has difficulty in using the eyes for 
close vision. It will be seen, from this definition, that the term 
presbyopia does not correspond to some well-defined condi¬ 
tion, as do the terms myopia, hypermetropia and astigmatism. 

This distance of eight inches, which has been fixed upon 
as the point of departure for presbyopia, is determined in an 
arbitrary manner, and yet it is not without reason. Some 
other distance could have been taken, and might, perhaps, 
answer equally as well (as seven and a half inches or eight and 
a half inches); but our great master, Donders, fixed upon eight 
inches, and it will be found that the greater number of authors 
use this same distance of eight inches. When the near point 
has receded to this distance, the individual first begins to 
experience difficulty in using his eyes for reading. Of course, 
a great deal will depend on the distance at which the person is 
accustomed to read or do his work. A man who is used to 
reading and holding his book at a distance of fifteen or eigh¬ 
teen inches will not feel the influence of age, or become pres¬ 
byopic, nearly as soon as one who is in the habit of holding 
his reading as close to his eyes as ten or twelve inches. The 
latter will become presbyopic much sooner than the former, 
and yet the condition of the refraction may be the same in both 
cases. 

SYMPTOMS OF PRESBYOPIA. 

As a person with emmetropic eyes gets along in years and 
approaches his thirty-fifth milestone, he instinctively, in his 
reading, begins to seek print a little larger than he has been 
accustomed to, and he is apt to hold his book a little farther 


PRESBYOPIA. 


316 

away from his eyes than formerly, and he looks for the best 
possible light on his work. These differences, however, may 
be so slight and the changes have been so gradual, as to have 
entirely escaped the patient’s attention. 

At forty years of age these changes have become more 
perceptible, and he begins to be conscious of the fact that he 
cannot see small objects as close or as well in a dim light as 
formerly; but, in spite of this, he manages to get along pretty 
well, and does not suffer any inconvenience from his eyes, 
and scarcely feels the need of any artificial assistance. 

At forty-five years of age these changes have all become 
intensified, and the fact is forced upon him that reading by 
artificial light is attended with more or less difficulty. He 
thinks that the books and newspapers are not printed in as 
-clear and legible type as formerly, and he complains that the 
light is very poor nowadays. In writing he finds difficulty in 
keeping on the pale ruled lines, but is one time above it and 
another time below it, because the line is really invisible. 
Letters like n and u are not sharply defined, and are scarcely 
distinguishable one from another. The head is thrown back 
and the book held at arm’s length, and the effort is made, and 
persisted in, to continue the reading. 

The strongest possible light is sought, and oftentimes the 
patient will place the lamp he is using between his eyes and 
his book. The light in this position serves a double purpose; 
it affords a greater illumination of the printed page, and at the 
same time the bright light, in shining into the patient’s eyes, 
causes the pupils to contract, and so lessens the circles of diffu¬ 
sion by shutting out the circumferential rays, and thus assists 
vision. 

The necessity for holding the book at so much greater 
distance causes the visual angle under which the letters are 
seen to be greatly diminished; consequently, the images 
formed upon the retina are smaller and cover fewer of the rods 
and cones of that membrane, and a strong light tends to in¬ 
crease the brightness of the retinal image, and thus, to some 
extent, to compensate for the smallness of the image. 

In studying the symptoms of presbyopia and noting the 
changes produced in the crystalline lens, and the effect of such 


PRESBYOPIA. 


317 


changes on the sight, we find that spherical aberration plays 
an important part, and it should be thoroughly studied and 
clearly understood. 

SPHERICAL ABERRATION. 

In spherical lenses the rays that pass through the periph¬ 
eral portions of the lens are brought to a sooner focus than 
those passing through the more central portions, or, in other 
words, there is an excess of refractive power in the marginal' 
portions as compared with the central portions. This excess 
increases with the distance from the center, and, therefore, the 
focal point for marginal rays is nearer than that for central 
rays. In such a lens there cannot be any common focal point 
for all the rays, and, this being the case, there cannot be a per¬ 
fect image formed. It is blurred because the conditions for a 
perfect image are not fulfilled. 

The use of a diaphragm, by covering the marginal parts 
of the lens, would greatly diminish the spherical aberration by 
shutting out all but the central rays; but though this would 
increase the clearness of the image, it would, at the same time, 
diminish its brightness, and, therefore, it can be used only 
when the light is very intense. 

Complete prevention of spherical aberration can be ac¬ 
complished only by increasing the refraction of the central 
portions of the lens, making it approximate that of the peri¬ 
pheral portions. 

Following out this idea, the prevention or correction of 
spherical aberration can be accomplished in two different 
ways, viz.: First, by increasing the curvature of the central 
portions of the lens; second, by increasing the density of the 
lens, and, at the same time, its index of refraction. It is by 
means of the first method that art is able to overcome spherical 
aberration; while it is chiefly by means of the second method 
(or, perhaps, by a combination of both) that nature prefers to 
accomplish the same purpose. It is an interesting study to 
observe the different methods adopted by art and by Nature to 
overcome the defects of spherical aberration. 

In regard to the first method (that is, the increase of the 
curvature of the central portions of the lens), it has been de- 


PRESBYOPIA. 


318 

termined by mathematical calculation that this increase of 
curvature must approximate that of an ellipse. It is justly 
considered one of the greatest triumphs of science to have been 
able to calculate the proper increase of this curve, and also of 
art to have carried out successfully the suggestions of science. 
Therefore, a lens, in order to make a perfect image, must be a 
segment, not of a perfect sphere, but of the end of an ellipsoid 
of revolution about its greater axis. 

An ellipse is an oval figure bounded by a regular curve, 
and is formed by the intersection of a plane and a cone, when 
the plane passes obliquely through the opposite sides of the 
■cone. An ellipsoid , in geometry, is a figure formed by the 
revolution of an ellipse about its axis. 

To overcome spherical aberration by means of the second 
method seems to be beyond the reach of art, that is, by an in¬ 
crease in the density of the central portions of the lens. It 
has not been found possible to so graduate the increasing 
density of glass, from the periphery to the center, in such a 
proportion as to exactly neutralize this aberration. But what 
is impossible with art is quite feasible in Nature, for it is by 
reason of this method that the human eye is spared the inter¬ 
ference with clear vision which spherical aberration would 
■cause. 

In the crystalline lens of the eye there is an increase in the 
density and in the refractive power of the lens from periphery 
to center. So marked and noticeable is this condition that the 
crystalline lens may be regarded as consisting of concentric 
layers, which increase in graded proportions in density and 
curvature from without inward, until when the central nucleus 
is reached, it is found to be a very dense and highly refractive 
spherule. The method of decreasing spherical aberration by 



Illustrating Spherical Aberration, or the Difference in the Focus of Central 
and Marginal Rays. 








PRESBYOPIA. 


319 


means of a diaphragm is also made use of in the eye, as the 
iris, by changing the size of the pupil, acts in this way by cut¬ 
ting off the marginal rays and allowing only the central rays to 
pass. 

It is evident to every student, and well known to every 
physiologist, that spherical aberration does not interfere with 
the clearness of the image formed in the human eye, and the 
result is accomplished by the methods mentioned above. 

This laminated structure of the crystalline lens has also 
another very important use, in addition to the correction of 
aberration as mentioned above. In a homogeneous lens, that 
is, one of the same solidity throughout, and without any con¬ 
centric layers of increasing density, the rays from the central 
part of the field of vision, that is, directly in front, would be 
brought to a perfect focus on the retina, and a clear image 
formed there; while the rays from the peripheral parts of the 
field of vision, that enter the eye under a certain degree of 
obliquity, would not be brought to a focus, and a more or less 
imperfect image would be the result. Consequently, the pic¬ 
ture formed by such a lens would be clear and perfect in the 
center, but very indistinct as the margin is approached. 

Now, this defect of a homogeneous lens is entirely cor¬ 
rected by the peculiar laminated structure of the crystalline 
lens. Therefore, the crystalline lens confers on the eye the 
capacity of distinct vision over a wide field, clear in the mar¬ 
ginal as well as the central portions of the field, and without 
changing the position of the point of sight. This capacity of 
the eye has been called periscopism, a term of great signifi¬ 
cance, a clear understanding of which will enable the optician 
to appreciate the advantages of the so-called periscopic lenses. 

SYMPTOMS OF PRESBYOPIA. 

As the consistence of the crystalline lens increases with 
the advancing years, the density of the peripheral portions of 
the lens approximates that of the nuclear portions, and, conse¬ 
quently, the defect of spherical aberration becomes more and 
more apparent. After reading the above paragraphs on the 
method which Nature adopts to overcome the defects of 
spherical aberration, that is, by an increase in the density and 


320 


PRESBYOPIA. 


refractive index of the more central portions of the lens, so 
that they may simulate that of the peripheral portions, the op¬ 
tician is in a position to comprehend and appreciate , the pres¬ 
byopic changes in the crystalline lens, and how this increase in 
the density of the peripheral portions of the lens defeats and 
destroys and neutralizes Nature’s method for the correction 
of aberration, and, consequently, the aberration becomes ap¬ 
parent because there remains no means for its correction, and 
this condition must be considered as one of the symptoms of 
presbyopia. Hence follow the advantages of a contraction of 
the pupil, which tends to overcome the aberration by shutting 
off the circumferential rays. 

In childhood and youth the density of the crystalline lens- 
diminishes from its center to its circumference, and, therefore, 
the peripheral rays are less refracted than they would be if all 
parts of the lens were of uniform density, and hence the cir¬ 
cumferential rays would be united at nearly the same point as 
the central rays. In childhood and youth, also, we find the 
pupil to be quite large, and this dilatation of the pupil permits 
the peripheral rays to pass, which will still be united at the 
focus of the central rays, on account of the gradation of density 
of the lens, as mentioned above. 

In good daylight, on clear days, and before the approach 
of twilight, the presbyope is still able to see fairly well; but he 
instinctively avoids fine print and seeks that which is large and 
clear. He finds there is some difficulty in reading and using 
his eyes by artificial light, and hence he seems disinclined to 
do much reading at night, except when compelled to do so, 
and then he seeks the brightest light and seats himself as close 
to it as possible. 

Pretty soon this difficulty in reading, which at first was 
apparent only at night, becomes noticeable also in the daytime, 
and now near vision at all times, and under all circumstances, 
becomes difficult and painful, and the individual has a well- 
marked case of presbyopia. There can be no longer any doubt 
that the sight has failed and that the eyes are not as good and 
strong as formerly, and the consciousness of this fact is at last 
forced upon the person, and he is compelled to acknowledge 
that his natural sight is not sufficient for his every-day needs. 


PRESBYOPIA. 


321 


This is rather a humiliating acknowledgment for some per¬ 
sons to make, as we find not a few people who take pride in 
boasting that they have arrived at middle age and still enjoy an 
undiminished acuteness of vision. Such persons imagine, and 
they easily persuade themselves that their imagination is the 
truth, that their constitution is so robust and their bodily vigor 
so great that the ordinary senile changes that affect others’ 
eyes cannot touch them; but they are finally brought face to 
face with the fact that they must have been mistaken, and that, 
after all, their eyes are not very much better than ordinary 
eyes. They only acknowledge themselves beaten when 
actually compelled to do so, because of their inability to any 
longer read, write, or do fine work with their unassisted eyes. 

This disposition to deny the existence, or at least the com¬ 
mencement, of aged sight is further shown by the expressions 
used by the patient in describing his experience. He will re¬ 
mark that “My eyes were always good until recently, and I 
think I must have strained them, or, perhaps, I caught cold in 
them; at any rate, I think they will be all right again in a few 
days.” Another one will say that his sight has been impaired 
by night-work or over-work. The next patient may say that 
his “sight was always excellent, and never began to fail until 
within the past few months,” or, that “the print which he was 
always accustomed to read now begins to blur and look dim.” 
While these excuses in some cases may be made in ignorance, 
yet, in the majority of cases, we are led to believe they are 
made with a desire to conceal the real cause, and to avoid lend¬ 
ing color to the fact that one is getting old enough to develop 
presbyopic symptoms. 

This interference with the clearness of reading and writ¬ 
ing and all close use of the eyes, which commences so 
gradually and insidiously, as described above, rapidly and 
steadily increases, and in the course of five or ten years dis¬ 
tant vision also becomes slightly impaired, that is, the sharp¬ 
ness of sight which the individual enjoyed in his earlier life 
becomes somewhat dulled. This impairment of distant vision, 
however, may not be so great as to cause any inconvenience; 
in fact, it may be so slight, and vision for distance remain so 
good, as to escape notice. 


322 


PRESBYOPIA 


If, for 'any purpose, such a person consults a competent 
optician and undergoes a careful examination of his eyes, the 
slightly defective vision or the loss of refractive power will be 
quickly discovered. Or, if by chance he puts on a pair of weak 
convex glasses (say, for instance, + .50 D. or + .75 D.), he 
finds everything appears clearer, and the details of the images 
of distant objects much more distinct, and their outlines a little 
more sharply defined. 

This is a condition of acquired hypermetropia, which sup¬ 
plants the natural state of emmetropia, the cause of which 
change of refraction is found in the condition of the crystalline 
lens, which grows firmer and of more uniform consistency, 
and, at the same time, becomes flatter. All of these changes 
tend to diminish the refractive power of the lens, and this 
causes the focus of parallel rays to fall behind the retina, which 
is precisely the same condition as occurs in hypermetropia. 
In the latter case the parallel rays focus behind the retina, 
because of the shortness or flatness of the eye-ball, while in 
presbyopia the focus falls behind the retina on account of the 
flatness of the lens. The result is the same in each case—the 
impairment of distant vision and its correction calls equally, in 
both cases, for a convex lens. 

In hypermetropia the symptoms indicating the approach 
of presbyopia appear at a very much earlier period of life, and 
in such cases it is usually accompanied with evidences of mus¬ 
cular fatigue and nervous and vascular irritation. 

In myopia, on the other hand, the appearance of presby¬ 
opia is delayed, the tardiness of its approach bearing a direct 
relation to the degree of near-sightedness. So that, in high 
grades of myopia, it is so much delayed and neutralized that 
it may never appear. In low degrees of myopia the patient 
enjoys good reading vision, without the need of glasses, until, 
perhaps, fifty-five years of age, after which he will begin to 
need weak convex glasses for reading, while he will continue 
to wear his concave glasses for distance. 

In the higher grades of myopia, where the far point lies 
within a limited distance, presbyopia cannot occur. Presby¬ 
opia depends upon a recession of the near point; but the near 
point can never recede beyond the position of the far point. 


PRESBYOPIA. 


3 2 3 


Now, presbyopia does not commence until the near point has 
receded beyond eight inches, and hence, if the myopia was so 
great that the far point was located at eight inches, or nearer, 
in such cases the near point could never recede far enough 
away to be classed as presbyopia. 

LOSS OF ACCOMMODATION. 

The child starts in life with a maximum amount of ac¬ 
commodation equal to about fifteen dioptres, and which does 
not commence to diminish until the tenth year of life. This is 
the earliest age at which accurate observations can be made; 
but the probabilities are that the infant possesses a still greater 
amount of accommodative power, perhaps equal to twenty 
dioptres, or even more. At forty years of age this has dimin¬ 
ished to five dioptres, or less; at forty-five years, to three 
dioptres; and at sixty-five or seventy years, scarcely one diop¬ 
tre of accommodative power remains. 

Of the fifteen dioptres of accommodation which the child 
of ten years is able to bring into action by the strongest effort 
of his volition, a considerable proportion (from two-thirds to 
three-fourths) may be lost before much inconvenience results 
in ordinary near vision. 

In emmetropia the distance of the binocular near point 
represents the number of dioptres of accommodation; hence, 
a near point of eight inches represents five dioptres of accom¬ 
modation, and, with the exercise of this amount of accom¬ 
modation, the smallest print in ordinary use can be easily de¬ 
ciphered by eyes of average visual acuteness at the said dis¬ 
tance of eight inches. When the power of accommodation is 
reduced to four dioptres, ordinary newspaper print can then 
be easily read at a distance of ten inches. 

With the loss of another dioptre of accommodation, that 
is, when three dioptres only are available for use, the near 
point has receded to thirteen inches, and now the reading of 
fine print becomes somewhat difficult, except under the most 
favorable conditions of good illumination and normal acute¬ 
ness of vision. The patient has now reached the age of forty- 
five; and very few persons attain this age without being made 
conscious of some defect in vision, and without seeking help 


3 2 4 


PRESBYOPIA. 


from convex glasses in reading or other fine work. When an 
emmetrope does begin to wear glasses, under forty years of 
age, it is generally due either to the exacting nature of the 
work in which he habitually employs his eyes or to the fact 
that his acuteness of vision is somewhat below the normal. 

When the patient has reached the age of fifty or fifty-five, 
and the power of accommodation has diminished to two diop¬ 
tres and the near point has receded to twenty inches, the book 
must now be held at arm’s length, which is such an incon¬ 
venient distance as to practically prohibit all use of the eyes 
for close work, except in the case of very unusually large 
print; although, even with this range of accommodation, a 
public speaker may be able to read fluently from a plainly 
written manuscript lying before him upon a reading-desk or 
table. 

The muscle of accommodation, as is the case with any 
muscle or organ of the body, loses tone and strength if not 
called into action to perform its usual function for any con¬ 
siderable length of time. Therefore, in cases of protracted and 
exhausting illness where the patient is unable to use his eyes 
during all that time, and the muscle of accommodation neces¬ 
sarily falls into a condition of inactivity, there may be a prema¬ 
ture development of presbyopic symptoms, which, as a matter 
of course, are interpreted as an indication for the immediate 
adoption of convex glasses. 

Another factor must be considered as tending toward the 
same result. These 'symptoms are due, not only to the total 
disuse of the accommodation during the illness, but also to 
the fact that the muscle of accommodation participates in 
the general bodily weakness occurring at that time. The im¬ 
pairment of vision varies very much in different individuals, in 
some cases causing but slight interference with the ordinary 
use of the eyes, while in other cases it is so pronounced as to 
prevent all use of the eyes for close work, unless some assist¬ 
ance is given in the shape of convex glasses. 

In such cases the glasses prescribed should not be too 
strong, but should be of the least power compatible with the 
use of the eyes under favorable conditions of illumination. 
The reason why preference should be given to the weaker 


PRESBYOPIA. 


325 


glasses is that the ciliary muscle should not be relieved of all 
incentive to action. The stronger the convex glass prescribed, 
the less the need of any exercise of accommodation. The 
weaker the convex lens prescribed, the greater the need of 
action of the accommodation. In other words, the convex 
lens, to the extent of its power, relieves the ciliary muscle of 
the exercise of its functions in the same degree. 

Now, in these cases, if the ciliary muscle was relieved of 
all necessity for action, it would remain in the same condition 
of disuse, and would, therefore, be placed under the most un¬ 
favorable conditions for regaining its lost strength. Whereas, 
if it was encouraged to act, and if it was called upon to perform 
only so much work as it could comfortably perform, such a 
method of management would tend to develop its powers, and 
would place it in a position the most favorable for regaining 
its original tone. Therefore, the patient should be encouraged 
in the hope that the eyes will soon grow strong again, and that, 
as the accommodative power increases with use, the glasses 
may soon be laid aside. 

Sometimes, in cases of this kind, it is possible to dispense 
with the aid of glasses, and in their place, and as a substitute, 
to use a solution of pilocarpine, and thus postpone the use of 
glasses for, perhaps, several years. The action of pilocarpine 
(which is the active principle of jaborandi) is to contract the 
pupil and stimulate the ciliary muscle, and its use twice daily 
for several weeks is very effective in again bringing the ac¬ 
commodation into use. 

The diminution of the range of accommodation and the 
recession of the near point which accompanies advancing 
years is a strictly physiological change, and must not be con¬ 
sidered as in any sense an unnatural condition, dependent, as it 
is, on the steady increase in the hardness of the crystalline lens, 
in consequence of which it becomes less and less capable of 
undergoing the necessary change in shape or increase in 
curvature which is required for the adjustment of the eye for 
near vision. 

As this increase in the density of the lens substance occurs 
in all eyes alike, irrespective of their condition of refraction, 
and as there are no exceptions to this change, it would seem 


326 


PRESBYOPIA. 


entirely proper to define presbyopia as the loss of accommoda¬ 
tive power incident to advancing years. Usage from time im¬ 
memorial and popular feeling on the subject, among opticians 
as well as the laity, have, however, associated the name with 
the special condition in which, as a result of increasing years, 
near vision becomes impaired, while distant vision remains 
unaffected. 

Viewing the subject in this light, presbyopia may be said 
to be an incident in the life history of every individual who 
reaches and passes middle life, with but few exceptions. The 
only exceptions are in cases of myopia, and then only when 
the myopia exceeds 4 D., because, in such eyes, the far point 
can never recede beyond ten inches, which is within the usual 
reading distance for fine print. Hence, the recession of the 
near point to an inconvenient distance, or the failure of vision 
for small objects close at hand, which are the characteristic 
and distinguishing marks of presbyopia, can never occur. 

Myopia and presbyopia in many ways seem directly an¬ 
tagonistic. In myopia only near objects are seen clearly, 
while distant vision is very indistinct; and in presbyopia dis¬ 
tant objects are seen clearly, while near vision is very indis¬ 
tinct. This apparent antagonism was recognized very early 
in the history of mankind, and because the nature and causes 
of the two conditions were not properly understood, myopia 
was regarded as the exact reverse of presbyopia for more than 
two thousand years. 

With our present knowledge on these subjects we are now 
aware that it is hypermetropia, and not presbyopia, that is the 
true opposite of myopia; but hypermetropia and presbyopia 
remained confounded until the middle of the present century, 
when the demonstration of the change in the form of the crys¬ 
talline lens in accommodation by Cramer and Helmholtz, and 
the masterly analysis of the phenomena of accommodation in 
its relation to the several anomalies of refraction by Donders, 
dispelled the cloud of obscurity in which the whole subject had 
been so long enveloped, and through which only momentary 
glimpses of the truth had been previously enjoyed by a few 
exceptionally acute observers. 


PRESBYOPIA. 


327 


PRESBYOPIA AS IT AFFECTS HYPERMETROPES AND MYOPES. 

A hypermetrope and a myope who had their defects prop¬ 
erly corrected and wore their glasses constantly, would begin 
to experience the disabilities of presbyopic vision at about the 
same age, and in nearly the same degree, as an emmetrope. 
About the age of forty, or soon thereafter, the hypermetrope 
discovers that his convex glasses are no longer quite sufficient 
for reading, and at the same time the myope finds that his con¬ 
cave glasses are becoming something of a hindrance in near 
vision, although, in both cases, the correcting glasses (in the 
first person convex and in the second concave) still continue 
to answer perfectly well for distance. A change to stronger 
convex glasses by the hypermetrope, or, rather, to an addi¬ 
tional pair of spectacles fitted with stronger lenses, and to 
weaker concave glasses by the myope, or, perhaps, even to a 
temporary removal of his glasses, are the remedies which now 
suggest themselves and which, sooner or later, must be 
adopted. 

With these changes in the glasses near vision again be¬ 
comes easy, but these reading glasses produce a correspond¬ 
ing diminution in the distinctness of distant vision. Conse¬ 
quently, in order that the individual may enjoy clear vision 
both for reading and distance, it becomes necessary to pre¬ 
scribe for the middle-aged ametrope two pairs of glasses, the 
one pair to correct the error of refraction and to be worn for 
distance, the other pair to be modified by the requirements of 
the accommodation and to be used for reading, which calls 
for stronger convex in hypermetropia and weaker concave in 
myopia. 

A hypermetrope who does not wear any glasses to correct 
his defect will experience the disabilities of presbyopia at a 
much earlier age than an emmetrope, after having passed 
through a more or less protracted stage of suffering from 
asthenopic symptoms. In myopia, on the other hand, if of 
not too high a degree, the defect may remain uncorrected 
■without any impairment of reading vision, but with the ad¬ 
vantage of retaining the reading power with the unaided eye 
until a more advanced age than in emmetropia. In the higher 


328 


PRESBYOPIA. 


grades of myopia the reading power is not impaired by the 
advance of years, but it is retained indefinitely. 

THE BINOCULAR NEAR POINT. 

The crystalline lens of the youth, on account of its soft¬ 
ness and elasticity, is easily changed in shape by but a slight 
effort of accommodation. The increasing firmness of the lens 
of the adult who has arrived at middle age presents a much 
greater resistance to be overcome in order to effect as much 
the desired degree of accommodative adjustment as is still pos¬ 
sible in presbyopia, and, therefore, calls for a relatively much 
greater effort of accommodation. 

In other words, the effort of accommodation required to 
adjust the eyes for a certain near point (say twelve inches) is 
much greater in middle age than in youth, while the effort of 
convergence is about the same in both cases. This disturbs 
and changes very materially the natural relation existing 
between the functions of accommodation and convergence; 
and the binocular effort of accommodation which is associated 
with convergence for the accustomed reading distance of from 
thirteen to fifteen inches finally represents nearly the total 
effort of accommodation which it is possible for the eye to 
exert by the strongest action of its will. That is, the binocular 
near point, as thus found, in reading, coincides very nearly 
with the absolute near point. 

When convex glasses begin to be used for reading, as is 
required in presbyopia, the distance of the near point increases 
very rapidly, and it is found that such reading power as may 
have been retained up to the time of the commencement of the 
wearing glasses is speedily lost, and then reading without 
glasses becomes almost, or entirely, impossible. Hence the 
usual experience of presbyopes is that, having once formed the 
habit of wearing convex glasses, their continued use has be¬ 
come imperative, and this is generally the case whether the 
person has commenced to wear the glasses rather earlier than 
absolutely necessary, or only after the need of them has been 
most urgently felt. 

It would, therefore, seem as if the too early use of convex 


PRESBYOPIA. 


329 


glasses in presbyopia was rather to be avoided, as entailing 
upon the wearer all the disabilities of presbyopia several years, 
perhaps, before they are normally due, and this is an argument 
that is often used by persons who, for various reasons, wish to 
defer the wearing of glasses as long as possible. 

But there are two sides to every story, and, in our opinion, 
the persons who are guided by the above reasoning as to the 
time when it is proper to begin to wear glasses are in error, 
and sooner or later they are made aware of it by the injury 
done to their eyes. The appearance of presbyopia is a physio¬ 
logical process, and the wearing of glasses for its correction is 
nothing more than supplying one of the natural needs of the 
system, and it is, therefore, the height of folly and presumption 
for weak man to attempt to thwart one of the inevitable laws 
of Nature; hence, the proper method of treatment is to recog¬ 
nize presbyopia early and to supply optical help liberally. 

A man of vigorous bodily constitution, with a corre¬ 
spondingly strong muscle of accommodation, may be able to 
defer the wearing of glasses for several years after the first 
symptoms of presbyopia begin to manifest themselves; but his 
ability to do this is accomplished only at the expense of a great 
strain upon his accommodation, which, perforce, in time 
weakens and destroys its integrity. 

If the glasses are worn quite early the strain is removed 
from the accommodation, and they supply the necessary con¬ 
vexity for focusing the rays upon the retina which aforetime 
was furnished by the ciliary muscle, and in proportion as this 
muscle is relieved of the necessity for extra exertion, in the 
same ratio does it lose its power to accomplish this act when 
deprived of the assistance on which it has been depending. 

THE RELATIVE STRENGTH OF ACCOMMODATION AND CONVER¬ 
GENCE. 

In studying the functions of accommodation and conver ¬ 
gence, and their relation to each other, and the effect of age 
upon them, one cannot help but be struck by the fact that 
accommodation begins to fail at the tenth year of life, and at 
the age of forty-five the impairment of its function has become 


330 


PRESBYOPIA. 


so marked as to interfere with the use of the eyes in the ordi¬ 
nary occupations of life; while convergence and the vigor of 
the muscles that control it (internal recti) continue unimpaired 
by the flight of years. 

The statement has been made on these pages, and is again 
repeated, that the functions of accommodation and conver¬ 
gence bear a distinct relation to each other, and, under normal 
conditions, are brought into exercise in the same proportion. 
For every effort of accommodation there is a corresponding 
employment of convergence, and for every effort of conver¬ 
gence there is a corresponding employment of accommo¬ 
dation. 

This correlation naturally existing between accommoda¬ 
tion and convergence is disturbed by the (several errors of re¬ 
fraction, and, also, by the senile changes that sooner or later 
overtake the eyes of every individual. In hypermetropia an 
excessive effort of accommodation is required to overcome the 
defect, in which class of cases the accommodation is in excess 
of the convergence. In myopia, on the other hand, but little 
effort of accommodation is required, while the nearness at 
which the object is necessarily held calls for a decided effort of 
convergence, and, in such cases, the convergence is in excess 
of the accommodation. This may lead on to the production of 
asthenopia, in the first case being accommodative, and due to 
exhaustion of the ciliary muscle; in the second case being 
muscular, and due to insufficiency of the internal recti 
muscles. 

In presbyopia there is found a like disturbance of the 
smoothness with which these two functions act in conjunc¬ 
tion, but the mechanism of the change is a little different. In 
these cases the disparity is due, not to the necessity for the 
exercise of one function in excess of the other, but to the 
actual weakening of the muscle of accommodation, in the first 
place, and to the increase in the density of the crystalline lens, 
which becomes less and less able to respond to the contrac¬ 
tions of the muscle, in the second place. Instead, then, of one 
function being in excess of the other, there is a deficiency of the 
accommodation as compared with the convergence. 


PRESBYOPIA. 


331 


PRESBYOPIA AND HYPERMETROPIA. 

Prebyopia is essentially an error of accommodation, and 
hypermetropia is essentially an error of refraction. This is the 
distinguishing difference between the two defects, which are 
so apt to be confounded on account of the similarity of their 
symptoms and on account of the same spherical convex lens 
being used to correct them both. 

The student should keep clearly in mind the distinctive 
difference between presbyopia and hypermetropia, as the inex¬ 
perienced optician is sometimes confused by the fact that the 
former sometimes apparently develops into the latter. This is 
not actually the case; but, as has been already explained in 
this chapter, in addition to the impairment of accommodation, 
which is the essential characteristic of presbyopia, the senile 
changes occurring in the eye all tend to a diminution of its 
positive refracting power, so that, finally, an emmetrope be¬ 
comes slightly hypermetropic (a condition of acquired hyper¬ 
metropia), a hypermetrope becomes still more hypermetropic, 
and a myope somewhat less myopic. 

This gives rise to the popular notion which is so preva¬ 
lent, that a person who is myopic in youth loses his defect of 
sight as he approaches middle age, and regains his normal 
vision. This is only partially true; the fact is, that while 
myopia is partially neutralized by the natural diminution of 
refractive power, it is only the very slight degrees of myopia 
that pass over to emmetropia. 

A small degree of hypermetropia, which sooner or later 
necessarily becomes absolute, is, in fact, the ultimate normal 
condition of all emmetropes, so that the time finally arrives 
when weak convex glasses will be required by such persons to 
clear up their distant vision; at such a time, and for similar 
reasons, the hypermetrope will require an increase in the 
strength of his convex glasses, while the myope will retain the 
same acuteness of distant vision with a weaker concave glass. 

This diminution in the refraction of the eye scarcely 
begins to be noticed at the age of forty-five, when the 
symptoms of presbyopia first begin to make their appearance. 
Even at sixty years of age it scarcely amounts to half a dioptre, 


332 


PRESBYOPIA. 


and it is not until the person reaches the allcxted three score 
years and ten, or over, that a + i D. lens is required to neu¬ 
tralize the acquired hypermetropia. 

LOSS OF ACCOMMODATION IN PRESBYOPIA. 

The deficiency of the power of accommodation depends 
on the age of the individual and the degree of presbyopia 
present in each particular case. In the early stages of pres¬ 
byopia, before the strength of accommodation has become 
markedly impaired, a weak convex lens only is required in the 
way of assistance, as the remaining accommodation is suffi¬ 
cient to complete the focusing of the rays of light upon the 
retina. 

The older the patient and the higher the degree of presby¬ 
opia present, the greater the deficiency of accommodation, 
until finally it is lost altogether, and then the power of the cor¬ 
recting convex lens must be increased until its principal focus 
will coincide with the distance of the reading point from the 
eye, and the focusing of the rays on the retina is accomplished 
entirely by the spherical convex lens, and without any assist¬ 
ance from the accommodation. 

In the different degrees of presbyopia there is a great 
variation between the amount of focusing that shall be done by 
the glasses and that which shall be accomplished by the effort 
of accommodation; and the fixing and determining how much 
shall be done by the lenses prescribed, and how much shall be 
left to the accommodation, opens up a wide field in the optical 
therapeutics of presbyopia for the exercise of the judgment of 
the optician and to test his skill. 

As has already been explained at considerable length, the 
near point represents the full power of accommodation which 
the patient is able to bring into play by the strongest effort of 
his volition, but this maximum of exertion of which the ciliary 
muscle is capable could not be sustained for any length of time 
for reading or sewing, or any other near work. The portion 
of the focusing that should be left for the accommodation to 
do must, therefore, be considerably less than the total amount 
of the accommodation power. 

It has been found that young persons under thirty years 


PRESBYOPIA. 


333 


of age cannot employ more than half Iheir total power of ac¬ 
commodation for any continuous use without suffering from 
eye-strain and symptoms of asthenopia. But those who are 
older, and whose accommodation is, on that account, greatly 
diminished, are able to use two-thirds of what remains without 
inconvenience. These proportions are only average, and 
although given to us by high optical authorities, they cannot 
be considered as inflexible; in fact, the optician, in his daily 
work, finds so many individual cases that vary widely from 
these figures and proportions that he is tempted to doubt the 
truth of them. However, in prescribing a lens for the correc¬ 
tion of presbyopia, in the absence of other indications he 
would be safe in following the rule to select a lens of such 
strength as would allow the patient to use two-thirds of his 
remaining accommodation for near work. 

The time of life at which presbyopia will occur in emme¬ 
tropic eyes is determined by the progress of the failure of ac¬ 
commodation, and the nearness of the point at which it is 
required to see clearly. The watchmaker and engraver are 
compelled to bring their work very close to their eyes in order 
to see the fine details of it, and are, therefore, oftentimes com¬ 
pelled to use a convex lens before the age of twenty. The 
laborer and the farmer, on the other hand, whose occupations 
are such as not to require any very close use of the eyes, fre¬ 
quently pass the age of fifty before the inconveniences of pres¬ 
byopia are noticed. 

These are examples of the two extremes; in the one in¬ 
stance demonstrating in a certain class of cases the early need 
of glasses, and in the other illustrating the age some persons 
may reach before the need of glasses is felt. For the mass of 
people, however, there is a daily necessity for more or less use 
of the eyes in near work, as in reading, writing and sewing, 
which is done in different cases at distances varying from 
twelve to eighteen inches; and, hence, very few people are able 
to pass the age of forty-five without seeking occasional help 
from glasses, or, at least, without feeling the need of such help. 
And for similar reasons, in emmetropic eyes, it is very rare for 
the disabilities of presbyopia to make their appearance until 
after the age of forty. 


334 


PRESBYOPIA. 


APPEARANCE OF PRESBYOPIA. 

The word presbyopia, therefore, is used to express a con¬ 
dition which depends on its approximation to, or departure 
from, a certain standard, which is arbitrarily fixed at different 
points by different authorities. In our judgment, the point of 
departure should be fixed at eight inches, and in accordance 
with this standard we have defined presbyopia in these pages 
as that condition of the eye in which the near point has 
receded beyond this limit of eight inches, and, as a conse¬ 
quence, the person has difficulty in using the eyes for close 
vision. 

On account of the difference in the ages of different per¬ 
sons when presbyopia makes its appearance, and because it 
does not correspond to some well-defined condition, as do the 
terms myopia, hypermetropia and astigmatism, Landolt 
seems very much dissatisfied with the word, as evidenced by 
the following extract taken from his manual of “Examination 
of the Eyes”: “But I frankly acknowledge that, in my 
opinion, it would be best to drop the term entirely out of our 
nomenclature, and to determine simply what lenses the patient 
has need of, not to see at a certain specified distance, but at the 
distance at which he is accustomed to use his eyes, or at which 
his work compels him to see. This can be done by taking 
account of his refraction and accommodation, as will be ex¬ 
plained further on.” 

Although Professor Landolt is an eminent scientist and 
distinguished ophthalmologist, and his opinion is at all times, 
therefore, to be respected, yet we cannot give his views as 
above without, at the same time, entering a vigorous protest 
against their adoption. We cannot see that he presents any 
convincing arguments why the term presbyopia should be 
dropped, and especially as he does not offer any other term as 
a substitute for it. In following out his views we would be 
deprived of the use of any term or word to describe that group 
of symptoms which are familiar to opticians everywhere as 
belonging to the condition we know as presbyopia, which, if it 
does not express a definite and well-cut condition, at least pre¬ 
sents to our mind a certain condition of the accommodation 


PRESBYOPIA. 335 

and a corresponding failure of near vision, which is measured 
in a certain way and corrected by a certain kind of lens. 

Landolt’s idea, as described above, is to determine simply 
what lenses the patient has need of, and he would, therefore, 
add to the definition of presbyopia as we have given it, or, 
rather, he would change it as follows: “Presbyopia finds its 
expression in the number of positive dioptres which it is neces¬ 
sary to add to the eye in order to procure a positive refracting 
power of 4.50 D.” 

It will be seen that this has reference to a near point of 22 
centimeters, or nine inches, instead of eight inches, as we prefer 
to make it. In order to see at this latter distance, there is 
evidently a positive refracting power required equal to 5 D. 
We have, on a former page, given a table representing the am¬ 
plitude of accommodation possessed by the eyes at different 
periods of life; a reference to which will show the student the 
surplus of accommodation possessed in youth, which grad¬ 
ually lessens until middle age shows it has passed over to a 
deficiency. 

Early in life, when the eye enjoys a great reserve of ac¬ 
commodative power, there is no difficulty in seeing at eight 
inches, and with a surplus of accommodative power left over. 
As age advances, and the accommodation diminishes, there 
is less and less surplus left over, and, finally, the time arrives 
when all the refractive power is required to adjust the eye for 
vision at eight inches, and there is no surplus of accommoda¬ 
tion in reserve. 

A little later on, and we find that the eye no longer pos¬ 
sesses a refractive power of 5 D., nor the ability, by any effort 
of its own, to see at the near point of eight inches, and then it 
evidently becomes necessary to increase the refractive power 
by means of a convex lens of such a power as will make it 
equal to 5 D. This lens, then, will measure the degree of the 
presbyopia. 


ANOTHER DEFINITION OF PRESBYOPIA. 

According to the line of reasoning just described, we can 
formulate a definition of presbyopia to be enunciated some¬ 
thing like this: Presbyopia finds its expression in the number 


336 


PRESBYOPIA. 


of the lens that represents the amount of the positive refracting 
power which it is necessary to add to the dioptric system of 
the eye in order to obtain a total positive refracting power 
equal to 5 D., which represents a near point of 20 centimeters, 
or eight inches. Or, in other words, the amount of presbyopia 
is expressed by that strength of convex lens which it is neces¬ 
sary to place before the eye in order to bring the receded near 
point back to eight inches. 

A study of the table and the diagram given on this and 
the following page will show the reader that, at the age of 
forty years, the emmetropic eye is scarcely able to exert a 
positive refracting power of 5 D., and its near point is begin¬ 
ning to recede beyond eight inches. From this time on the 
eye begins to be presbyopic. 

The degree of presbyopia in emmetropic eyes is equal to 
the difference between its positive refracting power, expressed 
in dioptres, and 5 D., which latter represents the point of 
departure for the commencement of presbyopia. The result of 
this sum of subtraction designates, at the same time, the 
number of the lens which the emmetropic eye requires for the 
correction of its presbyopia. 

By making use of this rule, the following table is obtained, 
always remembering that it refers to the presbyopia of emme¬ 
tropic eyes: 



Positive Refracting 


Age. 

Power. 

Degree of Presbyopia. 

40 

4.50 D 

5 D. — 4.50 D. = .50 D. 

45 

3-50 D. 

5 D. — 3.50 D. = 1.50 D. 

50 

2.50 D. 

5 D. — 2.50 D. = 2.50 D. 

55 

1.50 D. 

5 D. — 1 50 D. = 3.50 D. 

60 

.50 D. 

5 D — .50 D = 4.50 D. 

65 

0 

5 D. — 0 =5 D. 


Table compiled to show the degree of presbyopia at different ages, as ascer¬ 
tained by subtracting the positive refractive power at each age from 5 D. 

The positive refracting power of the eye gradually dimin¬ 
ishes with each year of life; but there are no evidences of this 
diminution of refractive power apparent during youth or early 
manhood, and not until after the fortieth year of life, when its 
deficiency must be supplemented by a convex lens, placed be¬ 
fore the eye. From this time on, the loss of refractive power 


PRESBYOPIA. 


337 


is marked and rapid, until the sixty-fifth year of life, when it 
is entirely lost, and then passes over to the negative side. As 
the refracting power of the eye, at this time, is no longer a 
positive force, but is now a negative quantity, for this reason it 
becomes necessary to add its value to the usual 5 D., in order 
to obtain the degree of presbyopia, instead of subtracting it, as 
in the years prior to this. 

It should be noted that the commencement of presbyopia 
is not deferred until the positive refracting power of the eye 
has been entirely lost, as at first sight might seem to be the 
case. Presbyopia does not commence where the refracting 
power stops. The o of the positive refracting power of the eye 
is not reached until the emmetrope is sixty-five years of age, 
whereas presbyopia, from being nothing at the age of forty 
years, begins to develop at this time, and increases at the rate 
of one dioptre for every five years, from forty up to sixty years 



This diagram represents the amount of refractive power at different ages by 
the height of the upright lines, illustrating the gradual diminution 
from ten years to sixty-five years, and then passing over to the 
negative side. The figures on the left indicate the 
number of dioptres of accommodative power, 
and the horizontal line of figures the 
different ages. 






























338 


PRESBYOPIA. 


of age. There is a break, at this time, in thie regularity of its 
increase, there being an advance of only .50 D. from sixty to 
sixty-five years of age, after which the same ratio of increase 
again holds good, that is, one dioptre fotr every five years, 
until the emmetrope has reached the patriarchal age of eighty 
years. 

AMOUNT OF AVAILABLE ACCOMMODATION. 

In studying the table that is given on page 336, it will 
be seen that the measure of the presbyopia is determined by 
that lens which it is found necessary to add to the positive 
refracting power of the eye, in order to raise it to 5 D. In 
other words, the eye must be able to possess this amount of 
accommodative effort, either naturally or artificially supplied. 
Now, with this amount of positive refracting power possessed 
by the eye, the question occurs, What proportion of it is avail¬ 
able for continuous use? 

One thing is certain, and that is, no eye is capable of call¬ 
ing into action the total amount of its positive refractive power 
and keeping up this exertion for any length of time. There¬ 
fore, only a certain proportion of the total amount of accom¬ 
modative power is available for use in the ordinary occupa¬ 
tions of life that call for near vision, and, while that proportion 
will vary greatly in individual cases, we may, perhaps, strike a 
general average, which will serve as a basis for our studies, 
and assist us in the proper understanding of this subject. 

Following out this matter along these lines, we have pre¬ 
pared the table on page 336, which shows the average dimi¬ 
nution of the positive refracting power for each five years, the 
amount of available accommodation for habitual work, and the 
strength of the lens that would be required to supplement this 
accommodation to permit of continuous near work at the 
usual reading distance, always presuming that we are figuring 
on an eye supposed to be emmetropic. 

It has always been customary to teach optical students 
that a patient is able to comfortably use, for continuous near 
work, only one-half of the total amount of the accommodative 
power he possesses. This is more likely to be the case with 
young persons, but it is not an unsafe rule to follow at all ages. 
However, during the presbyopic period cf life, when the 


PRESBYOPIA. 


339 


amount of available accommodation is very considerably di¬ 
minished, it is often found that perhaps two-thirds of what 
remains can be brought into continuous use without any per¬ 
ceptible inconvenience. 

The reading distance is usually considered to be at twelve 
to thirteen inches for the average number of persons; tall 
people may hold their book farther away, while short persons 
may find it more convenient to hold it closer. This distance 
may also be varied according to the calling of the individual; 
in those occupations which require very close vision, the habit 
is formed of holding the book near to the eyes. In those occu¬ 
pations, on the other hand, where the work is placed at arm’s 
length, or farther, the person oftentimes falls into the habit of 
holding his reading matter no closer than fifteen to eighteen 
inches. 

The distance at which the reading is held, or the work is 
done, is not alone a matter of convenience, or of stature, or of 
habit, but is directly dependent upon the amount of accommo¬ 
dation possessed by the individual, and available for con¬ 
tinuous use. 

As an illustration, we will take a man whose total amount 
of refractive power is 5 D. If this man is emmetropic, his age 
is about forty years, and he is just beginning to be presbyopic. 
This individual’s near point is at eight inches, at which dis¬ 
tance he is able to see only by the exercise of all of his refrac¬ 
tive power. Vision, at this point, is possible for only a few 
seconds or the fractional part of a minute. 

For sustained vision, such a person cannot use all of his 
positive refracting power, and, therefore, cannot see at eight 
inches; but he can use only a certain proportion of this 5 D., 
which will throw his reading distance further away. If he uses 
only one-half of his refractive power, that is, 2.50 D., his read¬ 
ing distance will be sixteen inches. If he is able to use two- 
thirds of it, that is, if his available accommodation is 3.34 D., 
his comfortable reading point would be as close as twelve 
inches. 

Hence, the proper reading distance for such a person 
would depend on two factors: First, the total amount of re¬ 
fractive power he possessed, and, second, the amount of 


340 


PRESBYOPIA. 


refractive power that could be considered as available for 
every-day use. As the majority of presbyopic persons have 
the ability to use and make available two-thirds of their ac¬ 
commodative power, we have calculated on that proportion in 
the preparation of the following table: 


Age. 

Dioptres of 
Accommodation. 

Dioptres 

Available. 

Lens 

Required. 

40 

4 50 D- 

3 D. 

•34 D- 

45 

3-50 D. 

2.34 D. 

1 D. 

50 

2.50 D. 

1.67 D. 

1.67 D. 

55 

1.50 D. 

1 D. 

2.34 D. 

60 

.50 D- 

•34 D- 

3 D. 

65 

0 

0 

3-34 D- 


In the consideration of this table, it should be borne in 
mind that the point of departure for the commencement of 
presbyopia is when the inherent refractive power of the eye 
falls below 5 D., and its correction consists in adding a lens of 
such strength as will restore its refractive power to 5 D. If 
we consider at this age that two-thirds of the accommodative 
power is available for habitual near work, then this 5 D. of 
total refractive power represents 3.34 D. of working refractive 
power, and this table is based on the principle of supplying a 
lens of such strength as will keep the amount of available 
accommodation always at this point of 3.34 D. 

The optical student who is just commencing the study of 
these subjects may, perhaps, be somewhat confused by the 
lenses .34 D., 1.67 D., 2.34 D. and 3.34 D. He will look over 
his trial-case very carefully, but he will be unable to find any 
lenses marked with these numbers. This will be discouraging 
to him, and apt to make him disgusted with the whole subject. 
But there is a very simple way out of the difficulty, and that is 
to substitute that lens found in his trial-case which is the next 
strongest to the one desired. 

For instance, in place of + .34 D. we substitute + .50 D.; 
in place of 1.67 D. we use 1.75 D.; for 2.34 D. we select 2.50 
D.; and for 3.34 D., 3.50 D. The difference between these 
lenses is so slight that this substitution can be made without 
materially affecting the result; the positive effect of the differ¬ 
ence between the lenses, that is, of a lens of + .08 D. and of + 
.16 D., is so small as to be scarcely appreciable. 

In comparing these two tables, it will be seen that there 


PRESBYOPIA. 


341 


is a very wide variance in the results obtained; but it must be 
remembered that they are worked out from a different stand¬ 
point and on a dissimilar basis. At any rate, we desire to 
emphasize the statement that these tables 
are made for the purpose of illustration 
alone, and that they should not be used 
as a basis for the prescription of glasses 
in any individual case. 

And the point might be emphasized 
right here that no table has ever been 
prepared, nor is it within the range of 
possibility that such a table can be con¬ 
structed, that should serve as a guide for 
the selection of glasses in presbyopia. 

Formerly the idea was prevalent among 
the people, and it was shared in by jew¬ 
elers and would-be opticians, that when a 
middle-aged person felt the need of 
glasses, all the information that was 
necessary for the making of a prescrip¬ 
tion was the age, and immediately the 
number of the glass would be forthcom¬ 
ing. A table was in use in which the 
different ages called for a certain number 
of lens, and the jeweler who possessed 
such a table considered himself well in¬ 
formed in optics and competent to sell 
and adjust spectacles. What a pity that 
such a dream of perfection must be de¬ 
stroyed! 

Inasmuch as presbyopia is due to an 
inability of the power of the accommoda¬ 
tion to adjust the eye for things close at 
hand, it manifests itself by impaired 
vision, or strain, when the eyes are used 
upon near objects. Such being the case, 
it is evident that the first indications of it 
will be noticed when the eye is called 
upon to look at very small objects, or 



Accommodative Ther¬ 
mometer. 

The figures on the 
left represent the num¬ 
ber of dioptres of re¬ 
fractive power, while 
on the right are mark¬ 
ed the different epochs 
of life, with the grada¬ 
tions of each from one 
extreme to the other. 















342 


PRESBYOPIA. 


when the light is poor and dim; or, in other words, when the 
act of vision is performed under such circumstances as to 
impose an extra effort, either on the accommodation or on the 
perceptive layer of the retina, as, for instance, when reading is 
continued after sunset and when twilight covers the earth, or 
when it is foolishly attempted by the pale light of the moon, as 
a thoughtless person is sometimes rash enough to do. 

When reading is carried on under such circumstances, the 
natural impulse of the person is to bring the object looked at 
closer to the eyes. The reason for this is twofold: In the 
first place, the near approach of the object yields a larger 
retinal image, and, in the second place, there results a corre¬ 
sponding increase in the illumination. Now, while the prox¬ 
imity of the object improves vision in the manner indicated, it 
at the same time imposes an increased effort upon the function 
of accommodation. This may be possible for a time, but only 
at the expense of a terrible strain on the eye. Sometimes this 
extra effort and marked strain may be kept up for an incredi¬ 
bly long time, and all the while the print is clearly seen and 
near vision seems unimpaired; but, finally, the endurance of 
the muscle of accommodation is exhausted, and it is no longer 
able to keep up the strain and maintain the focal adjustment 
of the eye for near vision, and then it suddenly relaxes, with 
the result that all distinctness and clearness of near objects are 
lost, the print blurs and runs together, and reading or sewing 
has to be discontinued. 

A moment’s rest, a shutting of the eyes, a pressing of the 
closed lids with the hands, gives the ciliary muscle an oppor¬ 
tunity to recover, somewhat, its exhausted strength, and read¬ 
ing again becomes possible for a little while. However, the 
blurring of the print soon occurs again, and this time after a 
much shorter interval than before, when the temporary rest 
and closing of the eyes must again be resorted to before the in¬ 
dividual can continue his reading. If, notwithstanding these 
difficulties in reading, the use of the eyes be persisted in, the 
intervals of clear vision become shorter and shorter, and the 
periods of forced rest grow longer and more frequent, until, 
finally, in spite of all desire to continue, the individual is forced 
to give up the attempt. 


PRESBYOPIA. 


343 


This blurring of the type and difficulty in reading become 
more noticeable when the general physical condition of the 
individual has, for any reason, fallen below par, as in cases of 
recovery from protracted illness, or, in less degree, in the latter 
part of the afternoon, or in the evening, when the body is 
wearied and exhausted by the day’s exertion. 

In cases such as we have been considering, instead of this 
actual failure of vision and utter inability to continue the use 
of the eyes, it is remarkable how some persons are able to read 
fairly well even for long periods of time, although, as a matter 
of course, it is only at the expense of a great deal of eye-strain. 
Although conscious of this strain, yet their ability to read is 
such that they put off the wearing of glasses as long as pos¬ 
sible, and, as a result of this neglect, the individual soon begins 
to complain of irritation or inflammation of the conjunctiva, 
and the eyes begin to present evidences of vascular congestion. 

When such persons catch cold it settles in their eyes, and, 
as a result, they are afflicted with frequent attacks of acute 
conjunctivitis, which, after a time, develops into a condition of 
chronic conjunctivitis, and they are troubled more or less con¬ 
stantly with smarting and burning of the lids, which symptoms 
are very much aggravated when the eyes are used for near 
work. 

There may be other symptoms of eye-strain sufficiently 
pronounced to constitute an indication for glasses in pres¬ 
byopia, but those above mentioned are the ones most fre¬ 
quently met with. Another very common symptom, and 
sometimes the only one complained of, is that the book has to 
be held at such an inconveniently great distance in order that 
the print may be clearly seen. 

These various symptoms and evidences of asthenopia are 
the cry of the eye for help and assistance, and, consequently, 
the cry will continue as long as such assistance is withheld, 
and as long as the demand for it exists. Therefore it is not 
the part of wisdom to disregard this cry, but the careful man 
should rather anticipate its appearance and endeavor to pre¬ 
vent its occurrence. 

This has reference more particularly to the assistance that 
can be afforded by glasses properly and scientifically adjusted; 


344 


PRESBYOPIA. 


but it is not always and not only the choice of glasses which is 
to be determined, but, in some cases, whether the symptoms 
which suggest the need of glasses may not point to one of 
those dangerous and insidious diseases of the eye which some¬ 
times make their appearance as middle life is reached. 

One of the most common of the morbid changes which 
come on at this age, and threaten the eye with a total loss of 
vision, is that dreadful disease glaucoma , which so often mani¬ 
fests itself to the patient and his friends (and, sorry to relate, 
sometimes even to his optician) for a considerable length of 
time solely by the one symptom of a rapid increase of what is 
considered “old sight.” 


GLAUCOMA. 

Glaucoma is a disease characterized by an abnormally in¬ 
creased intra-ocular pressure, and under this heading there is 
associated a group of symptoms which are to be referred in 
large part to this increased tension. Unfortunately, the symp¬ 
toms complained of are not always directly and distinctly sug¬ 
gestive of the disease, and, therefore, it is often overlooked in 
its incipiency. 

The patient complains of his vision becoming gradually 
impaired, and of nothing more, as a rule. This manifests itself 
by a steady and gradual recedence of the near point, and an 
early appearance and a marked increase of presbyopia. This 
is due to, and dependent on, a pressure on the ciliary muscle, 
which interferes with its action and prevents its contraction, 
and, therefore, the patient requires a much stronger convex 
glass for reading than his age would indicate, and there may 
even be a positive diminution of the refractive power of the 
eye, simulating a condition of acquired hypermetropia. 

In connection with this diminution of the amplitude of 
accommodation, there is an occasional appearance of colored 
halos around the flames of lamps and candles, sometimes ac¬ 
companied with attacks of fogginess of general vision, the 
duration of which may be but a few minutes, or they may last 
for several hours. Such attacks of dimness of vision are more 
apt to occur after a sleepless night or after a meal, and are 
sometimes accompanied with orbital pains. 


PRESBYOPIA. 


345 


It is a well-established fact, and one that is more or less 
familiar to all opticians, that the use of mydriatics should be 
religiously shunned in an eye with a tendency toward glau¬ 
coma, as the instillation of a single drop of an atropine solu¬ 
tion has been known to develop an attack of this dreadful 
affliction in a person who has a predisposition to the disease. 

An eye with a tendency toward glaucoma has its greatest 
enemy in a mydriatic, it acting on the eye as a most virulent 
poison does on the body, and, in fact, no more injury could be 
inflicted on the eye by stabbing it with a sharp knife than is 
produced by an acute attack of glaucoma superinduced by the 
use of a solution of atropine. 

For these reasons the optician should be extremely care¬ 
ful in ordering atropine to be dropped into an eye for the 
purpose of examining the condition of its refraction, especially 
in those persons who have reached or passed the age of forty 
years. The well-informed optician is constantly on his guard 
not to fall into such an error. 

SYMPTOMS OF GLAUCOMA. 

In view of the dangerous character of glaucoma, and its 
liability in the early stages to be confounded with presbyopia, 
it is a matter of importance to the educated optician that he 
should be familiar with the nature and symptoms and appear¬ 
ance of this disease, in order that he may be able to recognize 
it early, and that such a dangerous disease may not be con¬ 
founded with the physiological and natural change which we 
know as presbyopia. 

The commencement of the disease, the development of its 
different symptoms, and the course which glaucoma may run, 
present numerous variations, and for this reason a precise 
classification is almost impossible, or, at least, extremely diffi¬ 
cult. There are several varieties of the disease, but these show 
a great tendency to pass over into ea,ch other. The resemblance 
of these different forms is quite marked, being distinguished 
from the very commencement by certain characteristic symp¬ 
toms, and, although varying somewhat in their course, they 
all too surely lead to that hopeless condition of blindness in 
which the eye-ball is stony hard, the pupil widely dilated and 


346 


PRESBYOPIA. 


fixed, the refractive media clouded, the optic disk cupped, and 
the sight entirely lost. 

In studying the different varieties of glaucoma from a 
clinical point of view, we find that one class of cases is distin¬ 
guished from the commencement by more or less marked 
inflammatory symptoms; whilst another class of cases appears 
to be, to a great extent, free from inflammation. This, natu¬ 
rally, divides the cases of glaucoma into two principal classes, 
as follows: 

1. Cases attended with inflammatory symptoms. 

2. Cases in which there are apparently no inflammatory 
symptoms present. 

PREMONITORY STAGE OF GLAUCOMA. 

In the great majority of cases of glaucoma there is a pre¬ 
monitory stage of the disease, which is characterized by several 
or all of the following symptoms. It should be noted that at 
first these symptoms are only of periodic occurrence, and there 
is usually, in the interval between the attacks, a perfect inter¬ 
mission, or freedom from all trouble. It is in this premonitory, 
or initial, stage that the optician should be able to recognize 
the disease, not for any purposes of treatment, but solely that 
the recognition may prevent the catastrophe of dropping a 
mydriatic into an eye like this, and to distinguish it and differ¬ 
entiate it from presbyopia. In speaking of mydriatics in this 
connection, we have reference to solutions of atropine, duboi¬ 
sine, and homatropine; cocaine, on the contrary, is a mydriatic 
which does not increase intra-ocular tension, but, on the con¬ 
trary, seems to diminish it. 

i. Early Presbyopia or Rapid Increase of Any Pre-existing 
Presbyopia. There is recession of the near point and a diminu¬ 
tion of the range of accommodation, and also of the positive 
refracting power of the eye. As the persons attacked by glau¬ 
coma are mostly beyond forty-five and fifty years of age, some 
degree of presbyopia is generally already present; but it is 
found that this often increases in a very rapid and marked 
manner during the premonitory stage of glaucoma, so that the 
patient may be obliged, in the course of a few months, fre- 


PRESBYOPIA. 347 

quently to change his reading-glasses for stronger and 
stronger ones. 

This rapid increase in the presbyopia appears to be due, 
not so much to a flattening of the cornea through an increase 
in the intra-ocular tension, as to the action of this pressure 
upon the nerve supplying the ciliary muscle, thus causing 
paralysis of the latter. Some authorities have called par¬ 
ticular attention to the fact that hypermetropia very frequently 
occurs together with glaucoma. It seems probable that hyper¬ 
metropic eyes are more prone to glaucoma than others; and, 
again, hypermetropia may be developed in the course of the 
disease. The cause of this is somewhat obscure, but the most 
probable explanation is that it is dependent upon changes in 
the crystalline lens, by which its refractive power is very con¬ 
siderably diminished. 

In view of the fact that the majority of the eyes that are 
predisposed to glaucoma are either hypermetropic or astig¬ 
matic, or both, it naturally follows that the full correction of 
the ametropia should be made and that the glasses should be 
worn constantly. The eye should never be subjected to any 
strain, and should be used as little as possible by artificial 
light, and care should also be taken to avoid all strong emo¬ 
tional excitement, because there is certainly a large nervous 
element in the causation of this disease. 

2. Increased Tension of the Eye-ball. The method of as¬ 
certaining the degree of intra-ocular tension is as follows: 
The patient is directed to look slightly downward and to 
gently (not tightly) close the lids. The optician then applies 
both his forefingers to the upper part of the eye-ball behind 
the region of the cornea, that is, over the sclera. The pressure 
should not be applied directly to the cornea, as this seems to 
increase the tension. 

One forefinger is pressed slightly against the eye so as 
to steady it, whilst the other presses gently on the ball from 
the opposite side, and in this manner estimates the degree of 
tension, by ascertaining if it is soft and yielding, or hard and 
unyielding. 

It is important that the optician should make himself 


348 


PRESBYOPIA. 


thoroughly acquainted with the normal degree of tension, 
which experience he can gain by the examination of a large 
number of healthy eyes. In this way the tips of his fingers 
become familiar with the normal degree of tension, and he is 
thus the more readily able to detect any departure from this 
condition. If, in spite of this education of the tips of his 
finger, the optician should still be in doubt as to whether there 
is any increase of the degree of tension in any particular case, 
he can compare it with the patient’s other eye, if healthy, or 
with some other healthy eyes, and by this comparison he will 
be able to determine the increase of tension, if any exists. 
This is usually a simple matter and presents no difficulties, 
except in cases of oedema of the eye-lids or chemosis of the 
conjunctiva. 

In place of the fingers, instruments have been devised to 
estimate the degree of intra-ocular tension with the extremes! 
nicety possible. These instruments are called tonometers , but 
it must be admitted that the results obtained by their use are 
not sufficiently accurate to render them preferable to palpation 
by the fingers. Various forms of tonometers have been in¬ 
vented by different authorities, some of which have appeared 
to answer better than others. One of them is constructed on 
the principle of indicating the depth to which a minute pin 
connected with the instrument is pressed into the sclerotic, and 
also the force employed to produce the depression. A modifi¬ 
cation of this tonometer was afterward introduced, with which 
an impression or depression is made in the sclerotic with a 
given definite force, the depth and breadth and general shape 
of which can be accurately measured in all directions. 

It seems to us not only useless, but preposterous, to make 
these attempts to devise an instrument to perform something 
which is so easily and so much better accomplished by the 
fingers. In this instance, as in so many other cases, Nature 
affords us a better instrument than art can supply, yet discon¬ 
tented man tries to improve upon it. 

NOMENCLATURE OF TENSION DEGREES. 

Signs have been devised for designating the different 
degrees of tension of the eye-ball which will be found of some 


PRESBYOPIA. 


349 


use, not only in practice, but also in making and keeping an 
accurate record of the state of the tension of any particular 
eye and its variations, from time to time, as well as the ad¬ 
vantages of such a system in the reporting of a case. 

Gentlemen who have given attention to this subject have 
found it possible and useful to distinguish nine degrees of ten¬ 
sion of the globe of the eye, which, for convenience and accu¬ 
racy in recording cases, have been designated by special signs. 

“T” stands for tension and “Tn” for normal tension. 
The interrogation point (?) is used after a sign to indicate 
that there is some question or doubt as to whether the sign 
accurately expresses the condition of the tension, and, in many 
cases, this is as near definite as it is possible to describe it. 

The minus (—) and plus (+) signs preceding the letter 
T, are used to designate whether there i.s a diminution or 
increase of the natural tension, while the numerals following 
the letter T indicate the degree of diminution or increase. 

TENSION SIGNS. 

Tn. Tension normal. 

+ T. ?. A very slight possible increase of tension, with 
a large element of doubt as to whether there really is an 
increase or not. 

+ T. i. First degree of increase of tension, slight but: 
without any doubt. 

+ T. 2. Second degree of increase of tension. Tension 
is considerably increased, so that the fingers can make but a 
slight impression upon the coats. 

+ T. 3. Third degree of increase of tension. Extreme 
degree of increased tension, the eye-ball is of stony hardness,, 
so that the fingers cannot dimple it by the firmest pressure. 

_ T. ?. Indicates a slight possibility that the tension is- 

below normal, with very much doubt as to whether there really 
is any diminution of the natural tension. 

_ X. 1. First degree of diminished tension, the diminu¬ 
tion being slight but undoubted. 

_ X. 2. Second degree of reduced tension. Tension is- 

so markedly diminished that the eye-ball feels soft to the 
touch. 


350 


PRESBYOPIA. 


— T. j. Extreme degree of diminished tension. The 
ball of the eye is so soft and flabby as to allow the finger easily 
to sink into its coats. 

It might seem, for ordinary purposes, as if some of these 
signs were carried to the extremes of refinement; and so they 
are, for the optician. But for the oculist who desires to make 
an accurate record of his cases, in studying the nature and 
watching the course of various diseases of the eye-ball, that 
are under treatment, there must be some method of distin¬ 
guishing the tension, and these signs have as much precision 
as is, perhaps, attainable under the circumstances. 

At the same time, it must be understood that the applica¬ 
tion of these' symbols to the various degrees of tension, and 
their interpretation, depends very much on the observer, and 
waries greatly with different persons, because the impression of 
resistance ascertained by the fingers does not convey exactly 
the same idea to every observer’s mind. For instance, if a 
certain case of increased tension be examined by two different 
opticians separately and apart from each other, one may re¬ 
cord it, + T. i, and the other write it, + T. 2. Or, the same 
case of reduced tension may be recorded by two different ob¬ 
servers as, — T. ?, and — T. 1. Such being the case, a certain 
sign does not always represent a definite degree of increased 
■or diminished tension. 

It should also be borne in mind that the normal tension 
has a certain range or variety in persons of different age, 
build or temperament; and, also, according to varying tempo¬ 
rary states of the system as regards fulness and depletion. A 
little experience makes the optician familiar with these varia¬ 
tions, which can then be scarcely confounded with the ab¬ 
normal grades mentioned above. 

THE SIGNIFICANCE OF INCREASED TENSION. 

Increased intra-ocular tension, then, is the chief and es¬ 
sential symptom of glaucoma, in whatever stage or form the 
disease may present itself to the optician, although this 
increase of tension may not be present in the same degree at 
.all times. 


PRESBYOPIA. 


35 1 

In making an examination of the tension of a normal eye¬ 
ball according to the method described above, the optician will 
find that the eye-ball dimples slightly on pressure, and that a 
sensation of fluctuation is communicated to the fingers, such 
as is caused by a collection of fluid contained in a tight, hard 
capsule. The amount of this pitting or fluctuation varies ac¬ 
cording to the degree to which the eye-ball is filled with its 
humors, and, also, to some extent, according to the thickness 
of the sclerotic coat; and therefore, as stated above, it is not 
precisely the same in every eye, even though normal. 

In the premonitory stages of glaucoma, the tension of the 
eye-ball, though markedly increased, does not reach a very 
high degree. In families in which glaucoma is hereditary, a 
marked increase of tension is often met with even in early 
life, although the disease may not break out till a much later 
period of life, or even not at all. 

In such cases it is proper to regard this increase of tension 
as abnormal and as a predisposing element of glaucoma, more 
particularly if it be accompanied with hypermetropia and a 
diminution of the range of accommodation out of proportion 
to the defect. 

It has been supposed by some authorities that the in¬ 
creased degree of tension always precedes, for a longer or 
shorter period, the other symptoms of glaucoma; but other 
eminent authorities have met with very marked exceptions to 
this rule. Still, an increase in the tension of an eye-ball 
should always excite the suspicions of the optician, and should 
at once lead him to examine the eyes as to whether any other 
symptoms of glaucoma are present. Even if none are found, 
still the eyes should be carefully watched and the patient 
warned as to the possible appearance of glaucoma, that he may 
observe whether any other symptoms present themselves. 

The optician should also be on his guard against an error 
frequently met with, that a sense of fulness or tension within 
the eye, experienced by the patient, is any proof of increased 
intra-ocular pressure and hardness of the eye-ball. For this 
feeling of fulness has been known to exist without the slightest 
increase in the tension of the ball. 

Another common error is to suppose that all acute inflam- 


352 


PRESBYOPIA. 


mations of ithe eye are accompanied by an increase in the intra¬ 
ocular tension. If, however, the degree of tension should be 
found to be increased, it must be regarded as a dangerous 
complication which is 'to be carefully watched, lest it be the 
forerunner of an attack of glaucoma. 

3. Subjective Appearances of Light and Color as Evidenced 
by a Halo or Rainbow Around a Light . This is also a very con¬ 
stant symptom of the premonitory stage of glaucoma. When 
looking at a gas-light or candle-flame, the patient sees a 
colored halo or rainbow around the light, the outer edge of 
which is red and the inner bluish-green. 

This has been supposed to be due to an interference with 
the proper refraction of the rays of light, owing partly to slight 
opacities in the media, partly to the dilated pupil, and partly 
to some changes in the peripheral portion of the lens. 

This colored ring seen around the light, and which is such 
a marked symptom of glaucoma, is the more noticeable the 
more the pupil is dilated, and disappears when the patient 
looks through a pin-hole disk. The stooping position some¬ 
times brings it on, and as this position favors congestion of the 
blood-vessels, we must regard the latter as a possible factor in 
the causation of the ring. 

4. Ciliary Neuralgia . A very marked symptom of glau¬ 
coma is pain, more or less acute, both in the eye and in the 
forehead and temples, radiating over the corresponding por¬ 
tion of the head and passing down the side of the nose. The 
pain sometimes occurs at a very early period of the disease, 
but occasionally it is not felt until a later stage, and, in some 
very rare cases, it may even be entirely absent. The pain is 
often very violent. 

These severe neuralgic pains are undoubtedly due to 
pressure upon the ciliary nerves in the sclerotic, caused by the 
more or less sudden increase of tension. Sometimes this 
neuralgia extends to other branches of near-by nerves. 

5. Dilatation and Sluggishness of the Pupil. When the 
pupil of an eye affected with the premonitory symptoms of 
glaucoma is compared with the pupil of the other eye (suppos- 


PRESBYOPIA. 


353 


ing it to be healthy), it will be found somewhat dilated and 
sluggish (or even immobile), and reacts but slightly, if any, 
to the stimulus of light. 

This sluggishness of the pupil is a well-marked symptom 
of the premonitory period of glaucoma, but the dilatation of the 
pupil is not so pronounced then as in the advanced stages of 
the disease, when it becomes widely dilated and quite immova¬ 
ble. The dilated pupil is sometimes oval in shape, with its 
long axis vertical. 

This condition of the pupil may be due to interference 
with the function of the retina, to anaemia of the iris, and to’ 
paralysis of the ciliary nerves supplying the 'iris, all of which, 
in turn, are the direct result of the increased pressure. 

6. Periodic Dimness of Vision. The patient is troubled 
with occasional attacks of temporary obscuration of sight. At 
such times surrounding objects appear veiled and indistinct, 
and as if they were shrouded in a gray fog or smoke. The 
degree of dimness varies considerably, as does also the dura¬ 
tion of the attacks; sometimes they last for only a few minutes, 
and again they may persist for several hours. 

These attacks may only cause a slight contraction of the 
field of vision, but generally there is indistinctness of certain 
portions of the field. These attacks are usually caused by dis¬ 
turbances in the circulation of the eye, producing a temporary 
cloudiness of the aqueous and vitreous humors. Similar ob¬ 
scurations may be caused and imitated by pressure upon the 
healthy eye. 

The impairment of vision is, perhaps, not so much due to 
the direct pressure upon the retina, as in the interference with 
the circulation, the fulness and stagnation of the veins, and the 
emptiness of the arteries. The truth of this seems to be proven 
by the fact that attacks of dimness of vision can be brought on 
by anything that causes congestion of the j^od-vessels of the 
eye, as, for instance, a full meal, great exJBment, long-con¬ 
tinued stooping, violent exercise, etc. 

7. Cloudiness of the Aqueous and Vitreous Humors. The 
aqueous humor is often found slightly but uniformly hazy. 


354 


PRESBYOPIA. 


rendering the structure of the iris somewhat indistinct, and- 
causing a slight change in its color. The vitreous humor, in 
like manner, becomes uniformly a little clouded. This hazi¬ 
ness of the humors is very variable in its duration and degree, 
in some cases being so slight as to be hardly perceptible, and 
in other cases so marked as to render the fundus invisible 
through the ophthalmoscope. 

The cause of the cloudiness is an exudation, partly in¬ 
flammatory in its character, and partly the result of venous 
stasis. It may come on frequently, only lasting a few minutes, 
or it may be less frequent and last longer. 

8. Venous Congestion. The congestion of the ciliary veins 
is usually slight during the premonitory stage of glaucoma, 
but they become tortuous and dilated in the later stages of the 
disease. An ophthalmoscopic examination shows the retinal 
veins also to be enlarged, with sometimes a venous pulsation. 

9. Arterial Pulsation. This may be seen on the optic disk 
by means of the ophthalmoscope, and is always to be con¬ 
sidered a sign of disease. This pulsation is a symptom of 
great importance, and it should be noted that it never occurs 
beyond the margin of the disk. 

10. Contraction of the Field of Vision. This contraction 
of the field usually commences at the inner or nasal side, and 
extends thence toward the center and also above and below, 
until, finally, in the later stages of the disease only a small 
portion of the field remains at the outer side. 

11. Diminished Transparency of the Cornea. The cornea 
loses its brightness and polish and its surface takes on a pe¬ 
culiar steamy appearance, resembling a piece of glass that had 
been breathed on. This is due to oedema of the corneal tissue 
and its epithelium by infiltration into them of the intra-ocular 
fluids by reason of the increased tension. 

12. Diminution of the Depth of the Anterior Chamber, from 
pushing forward of the lens and iris. 

13. Cupping or Excavation of the Optic Disk. In the later 
stages of the disease, if the refractive media are sufficiently 


PRESBYOPIA. 


355 


clear to allow of an ophthalmoscopic examination to be made, 
and if the pressure has been severe enough to produce the 
change, the disk is seen to be cupped or excavated. The 
whole surface of the disk is cupped, and even though the exca¬ 
vation is but slight, the edge is always abrupt and precipitous, 
and it may even overhang the cup. 

DIAGNOSIS OF GLAUCOMA. 

We have given considerable space to an enumeration and 
description of the symptomatic indications of glaucoma, be¬ 
cause it is important that the optician should be able to recog¬ 
nize this dreadful disease, not for purposes of treatment, but 
in order that he may not confound it with presbyopia, and thus 
fall into the fatal error of prescribing glasses for a disease 
which needs the treatment of a surgeon. This is an error that 
has occurred on more than one occasion, but it is one that 
should never occur \in the practice of an educated optician. 

The popular prejudice that has arisen, and not entirely 
without cause, as to the pernicious effects of wearing convex 
glasses which are too strong, can be traced to the fact that one 
of the premonitory symptoms of glaucoma, as mentioned 
above, is a rapid failure of the accommodation, and a conse¬ 
quent frequent demand for stronger and stronger glasses. 

As glaucoma is more apt to occur at the presbyopic 
period of life, before the etiology and pathology of this disease 
were thoroughly understood, opticians met with many cases 
that came to them every two or three months for stronger and 
stronger glasses, which sufficed for a little while; and then, in 
turn, had to be changed for still stronger ones, until, finally, 
glasses were no longer of any use, and the patient became 
almost entirely blind. What more natural than for the opti¬ 
cian to associate the use of the strong glasses with the blind¬ 
ness, in the relation of cause and effect? 

The lessons to be learned from this error are, in the first 
place, that strong glasses, in themselves, are not necessarily 
harmful, although they may produce some discomfort if they 
disturb the natural relation that should exist between the func¬ 
tions of accommodation and convergence. Evidence of the 
harmlessmess of the continuous use of a single strong convex 


356 


PRESBYOPIA. 


lens is seen in 'the case of watchmakers and engravers, among 
whom the wearing of such a lens is a necessary condition of 
their occupation, and who do not seem to suffer therefrom, but 
rather appear to enjoy an enviable immunity from diseases 
of the eye. The fact is, that the habitual exercise of the eye 
upon fine work, as, also, the normal use of any organ of the 
body, tends to the development and to the preservation of its 
powers. 

The second lesson to be learned is, that the premature 
demand for strong glasses may arise from the existence of 
some diseased condition; and hence, the circumstance that 
glasses are required at an early age, or that stronger and 
stronger ones are called for at short intervals by a person not 
originally hypermetropic, thus proving the abnormally rapid 
failure of the accommodation, should be a reason for placing 
the optician on his guard against the possible approach of this 
dread disease, glaucoma, and for suggesting the advisability 
of seeking the skilled advice of a physician in time. 

All of the symptoms mentioned above are not present in 
every case of glaucoma, and many of them may be so slight as 
to escape detection in the premonitory stages of the disease, 
when it falls under the observation of the optician, as being, 
perhaps, only an aggravated condition of presbyopia. The 
intensity of the symptoms will, naturally, vary with the severity 
of the attack, becoming very marked and extreme in sharp 
attacks of the disease, when, of course, it is beyond the stage 
at which it may be mistaken for a symptom of presbyopia, 
and when it is no longer difficult to make an accurate diag¬ 
nosis. 

RECOGNITION OF GLAUCOMA. 

When a patient presents himself to an optician, after the 
lapse of only a few months, with the statement that his glasses 
no longer suffice, and with the request for stronger ones, and 
when the optician finds there has been a sudden and rapid 
diminution of the amplitude of accommodation, and when the 
patient complains of the occasional appearance of colored 
halos around the flames of lamps or candles, accompanied with 
attacks of fogginess of the general vision, then the optician 
must be on the lookout. 


PRESBYOPIA. 


357 


If these symptoms are accompanied with severe orbital 
pains, slight opacity of the aqueous humor, and sluggishness 
of the pupil, or an immobile pupil partly dilated, the optician 
must be still more on the alert. 

If, in addition, the eyes present all the appearances of an 
acute inflammation with discoloration of the iris, and a steamy 
cornea insensible to touch, and if the acuteness of vision is 
very much impaired, then let the optician beware. 

If the optician be on his guard for these symptoms, he 
cannot fail to recognize them, when present, in any case that 
falls under his examination; and, when recognized, the well- 
read man understands their import, and is in a position to give 
intelligent advice, without falling into the error of prescribing 
glasses for a supposed case of presbyopia. This timely advice 
may be the “stitch in time” that saves the eye from the second 
and third stages of glaucoma and from blindness. 

No more telling argument could be used by physicians 
against the advisability of allowing opticians, or any one out¬ 
side of the medical profession, to prescribe glasses for defective 
vision, than to find a case of irremediable blindness as the 
result of glaucoma which had been in the hands of an optician 
and treated as a case of presbyopia, and thus the real nature 
of the trouble never discovered until the sight had been irre¬ 
parably destroyed. 

Such an occurrence would react against the individual op¬ 
tician who was the unfortunate cause of it, and against opti¬ 
cians as a class, while, at the same time, the innocent sufferer 
would pay the penalty with his sight, which, once lost, could 
never be regained. This is a hard lesson for any one to learn 
from actual experience, and we trust no reader of The Opti¬ 
cian’s Manual may ever be called upon to pass through such 
an experience. 

Of course, no optician would knowingly make such an 
error, or willingly place in jeopardy the sight of one of his 
patrons, neither should the patient be compelled to run the 
risk of having the one chance of the restoration and preserva¬ 
tion of vision denied him by the lack of knowledge of the opti¬ 
cian whom he consults, and in whom he places his confidence. 


358 


PRESBYOPIA. 


JEALOUSY OF THE MEDICAL PROFESSION. 

Physicians and oculists are annoyed and alarmed, more 
than they will admit, at the encroachments made upon what 
they consider their territory by the educated and graduate op¬ 
ticians of to-day. There is a large and ever-increasing number 
of persons who have their eyes examined and glasses adjusted 
by opticians, and without the advice of a physician. Middle- 
aged persons who needed glasses only for the correction of 
presbyopia, always purchased them of opticians or jewelers, 
and formerly the trade of the optician was confined to this 
class of cases. 

But in recent years the optician has developed beyond a 
mere seller of spectacles; he has shown a laudable desire to 
educate himself, and to adjust spectacles from a scientific 
standpoint. Not only this, but he has familiarized himself 
with the methods and instruments necessary for a thorough 
examination of the physical, as well as the refractive, condition 
of the eye. As a consequence of this more complete prepara¬ 
tion for his work, the field of the optician has enlarged con¬ 
siderably, and his practice is no longer limited to the sale of 
spectacles for the relief of aged sight, but extends to the suc¬ 
cessful correction of all the optical defects to which the eye is 
subject, and even to the relief of complicated muscular 
anomalies. 

This, naturally, has aroused the antagonism of the medical 
profession, inasmuch as it has diverted a large stream of 
profitable trade from the office of the oculist into the parlor of 
the optician. There is an old saying that a man can be touched 
or influenced through his pocket-book more quickly and po¬ 
tentially than by any other means, the truth of which remark 
is very aptly exemplified in the attitude assumed, and enmity 
shown, by the medical fraternity toward the optical profession. 
This is exhibited, not only by the physician in his daily walk 
and talk, but is reflected on the pages of the various medical 
journals. 

The position physicians assume is that no one but a 
medical graduate is competent to prescribe glasses, and, there¬ 
fore, they feel it their duty, in the interest of the public, to 


PRESBYOPIA. 


359 


warn people against the very great danger of wearing any pair 
of glasses not prescribed by an oculist, and they lose no 
opportunity to give opticians a “black eye.” 

A CASE IN POINT. 

The following is an abstract from an editorial article in a 
leading medical journal: 

“The observation of careful ophthalmologists indicates that 
many individuals are given glasses which are worse than 
useless, and that, in some instances, so much valuable time is 
lost that subsequent intelligent treatment is of no avail—some 
of these are cases in which, if a proper ophthalmoscopic ex¬ 
amination had been made and suitable drugs had been pre¬ 
scribed, the patient would have retained vision. 

“Such a case recently occurred in this city, where a young 
woman found her vision failing, and, on the advice of her 
friends, went to an optician, who examined eyes without 
charge, sold her a pair of glasses, and sent her home with the 
assurance that all she needed was Test glasses.’ A few weeks 
passed by, during which her vision grew gradually worse. 
Another consultation with the optician was held, and another 
pair of glasses was prescribed. This performance was gone 
through with twice more, with four changes of glasses in six 
months ; and by this time the patient’s vision was reduced to 
counting fingers at a distance of five feet. At this time an 
ophthalmoscopic examination revealed the unfortunate fact 
that the patient had post-neuritic atrophy of both optic 
nerves.” 

This is an exceptional case, and, if it occurred just as re¬ 
lated (which we have no reason to doubt), it is much to* the 
discredit of the optician implicated. No man who carefully 
reads The Manual and follows its teachings could fall into 
any such grievous error. An optician, competent to examine 
eyes and adjust glasses (and none other should be allowed in 
the business), could not fail to see there was some unusual and 
serious trouble in this young woman’s case, which called for 
attention other than the fitting of glasses. No reader of this 
book would assure such a patient that all she needed was rest 
glasses. 


360 


PRESBYOPIA. 


In cases of defective vision, the optician must discriminate 
between those which can and those which cannot be helped by 
glasses. This divides all cases into two great classes, and we 
have indicated how the optician can determine to which of 
these classes his case belongs. In the first class of cases he is 
justified in assuming the sole management; while, in the 
second class of cases, our teaching has been that an ophthal¬ 
moscopic examination should always be made by the optician 
himself if he is prepared to make it, or by an oculist to whom 
the patient may be referred by the optician. By following out 
this line of procedure, the optician saves his reputation and 
the patient saves his sight, while the medical profession is 
saved much needless anxiety about the danger of wearing 
glasses prescribed by an outsider. 

Another case mentioned in this article is that of a little 
girl who was found wearing concave glasses given her by an 
optician, when an examination showed that she was exceed¬ 
ingly far-sighted. We admit this is a serious error, and one 
that should not, and will not, occur in the practice of a careful 
and competent optician. 

ANOTHER UNFORTUNATE CASE. 

The previous case, where concave glasses were given in 
hypermetropia, recalls a case whose history was given in a 
widely-read medical journal, and the result severely commented 
upon, to the discomfiture of the prescribing optician. 

The patient was a young lady aged twenty-five, who 
stated that four weeks previously she noticed a haziness in the 
right eye, followed, iin a few days, by floating black spots, and 
later she suddenly lost the sight of the eye in question. This 
lady had always been near-sighted, and only a short time 
before the above-mentioned symptoms made their appearance, 
a jeweler (who happened to be her brother) changed her 
glasses for stronger concave ones, at which time she was able 
to read and see clearly. 

An oculist, who was now consulted, found the vision of 
the right eye was for fingers only, while the left eye was barely 
able to perceive large objects. An ophthalmoscopic examina¬ 
tion revealed a detachment of the retina in the right eye and a 


PRESBYOPIA. 


361 

posterior staphyloma in the left. The ordinary treatment for 
detachment of the retina was instituted, but a consultation 
with a view to operation was refused. The result is that she 
is blind in the right eye from the detachment of the retina, and 
the left eye for a long while had been useless. 

The oculist argues that, if an ophthalmoscopic examina¬ 
tion had been made at the time the glasses were changed and 
when the unpleasant symptoms first made their appearance, 
the condition of the fundus of the eye would have shown suffi¬ 
cient indications of danger to have warranted putting the case 
under treatment before irreparable damage had been done, and 
thus the only chance for successfully treating the case was lost. 
Hence, the ground is taken in the article that the trained phy¬ 
sician (the oculist) is the only proper person to prescribe 
glasses, and not the optician or the jeweler. 

The medical journals say it would not be hard to multiply 
illustrations of the injurious effects of prescribing glasses in 
cases like the above. At the same time, they admit that this 
practice does not extend to all opticians, but that there are a 
few honorable exceptions, who refuse to furnish glasses when 
they discover visual defects. However, they think that too 
many dealers in spectacles have no compunctions about usurp¬ 
ing the province of ophthalmologists, and yield to the tempta¬ 
tions of profit and the unwise solicitations of their customers 
to do what they have no right to do. 

One of the articles referred to closes as follows: “If the 
prescriber of glasses possesses the skill to use the ophthalmo¬ 
scope, and is able, also, to correctly interpret the various 
shades of normal and subnormal retinae and optic nerves, 
vitreous and choroid, then I should be the last one to deny him 
the privilege of prescribing glasses, be he oculist or optician. 
Lacking this special knowledge and skill as a refractionist, I 
would be the first to sound the warning to him personally, and 
use his ignorance in an impersonal manner to warn others.” 

This is an honest expression, and it is the only proper 
view to take of the whole matter under the circumstances men¬ 
tioned; but it must be remembered that the influence of the 
various optical colleges, the optical teachings of journals like 
The Keystone, and the progressive spirit manifested by op- 


362 


PRESBYOPIA. 


ticians everywhere to study their profession from a scientific 
standpoint, have served to place the optician of to-day so far 
beyond his brother of twenty-five years ago that there can be 
no comparison between them; and, therefore, while the cases 
narrated above, and the deductions and arguments of the 
various medical journals thereon, might hold against the back- 
number optician of a score of years ago, they cannot apply, as 
a class, to the studious and well-informed optician of to-day. 

OCULIST VS. OPTICIAN. 

The medical profession are willing to allow no one to fit 
and prescribe glasses who is not competent to use the ophthal¬ 
moscope and to interpret what it reveals, and they thus think 
they are shutting out all except physicians. The fact is, that 
not very long ago this instrument was reserved for physicians’ 
use alone, and it was considered entirely beyond the optician’s 
province. But the time has arrived when opticians, as a class, 
are demanding instruction in the use of the ophthalmoscope, 
the employment of which they claim as one of their privileges. 

That an ophthalmoscope should be included in the outfit 
of the well-prepared optician, there can be no doubt; but its 
exact status is still a matter of question. Some opticians seem 
to have such an exalted opinion of this instrument as to think 
it possesses some mysterious power by which to reveal to them 
the particular glass required by each individual case. But 
perhaps the best advice to the optician employing an oph¬ 
thalmoscope is not to use it for refraction purposes, that is, 
not to attempt to measure the optical defect and prescribe 
glasses by means of the ophthalmoscope. 

It would be well for opticians to regard this instrument 
simply as an aid in the examination of difficult cases. Where 
there is great impairment of vision, and where there is diffi¬ 
culty in restoring vision by the trial-case, then, in such cases, 
an ophthalmoscopic examination is of value to show the con¬ 
dition of the refractive media, of the optic nerve and retina, 
and of the whole interior of the eye, and to reveal the presence 
of any pathological condition that may be present, thus ex¬ 
plaining the cause of the impaired vision, and indicating 
whether glasses or medical treatment is needed. 


PRESBYOPIA. 


363 


THE PROVINCE OF THE OPTICIAN. 

While refraction work comes legitimately within the prov¬ 
ince of the qualified optician, no one should be considered an 
optician, and, therefore, no> one should be allowed to adjust 
glasses, who has not received special instruction in this work. 

For this reason it is imperative that the intending optician 
forsake, as being imperfect and unsatisfactory, the old way of 
acquiring a knowledge of the optical business by service be¬ 
hind an optician’s counter, and by picking up such a smatter¬ 
ing of the science of adjusting glasses as may fall from his 
employer’s lips, or as may be observed from a careful watching 
of his methods. 

THE VALUE OF INSTRUCTION IN OPTICS. 

While such an experience must not be decried, nor its 
practical value underestimated, the intending optician must 
convince himself of the necessity of adding to and supple¬ 
menting it by classified instruction in the theoretical and scien¬ 
tific principles underlying the science of optics, and including 
the entire field of study covering the subjects in which an opti¬ 
cian should be particularly interested. The field is so extensive 
as at first sight to be appalling, but this should only serve to 
stimulate the student in his efforts to comprehend it and famil¬ 
iarize himself with it. 

The optical student must first acquire a knowledge of the 
anatomy of the eye and the physiology of vision, then of the 
principles of optics, the laws and properties of light, its refrac¬ 
tion by transparent media of different density, and an acquaint¬ 
ance with the various kinds of lenses, and their action on rays 
of light that pass through them. With these matters thor¬ 
oughly understood as a broad and firm groundwork, the re¬ 
fraction and accommodation of the emmetropic eye should 
then be considered and carefully studied. 

THE AIM OF THE OPTICIAN. 

The ultimate aim of all the studies of the optician is to 
learn to recognize and correct the various forms of ametropia, 
and, therefore, while it is desirable for him to be well read and 


364 


PRESBYOPIA. 


well informed on all matters pertaining to his business, it is 
still more important for him to be able to put his theoretical 
knowledge into practical use. It is possible for a man to 
comprehend the theory of a science and yet be unable to put 
it into practical effect. 

The tendency of modern teaching, from the class-room of 
the primary school to the lecture-hall of the university, seems 
to be to cram knowledge into the scholar’s cranium by the 
stuffing process, leaving to> Nature, or to accident, or to en¬ 
vironment, or to it he scholar himself, the development of the 
ability to make practical use of his knowledge, if, perchance, it 
is ever developed. 

ABSTRACT KNOWLEDGE DOES NOT ALWAYS MEAN SUCCESS. 

Many instances of this can be found in the case of medical 
men. It is no trouble to point out physicians of talent and 
large scientific attainments, who are not nearly so successful 
as some of their less-educated brethren; the reason being that 
the one is too scientific and loves science for the sake of 
knowledge alone, while the other is more businesslike, and is 
interested in science only so far as he can make practical use 
of it. This is likewise true of opticians; it is possible for a 
man to thoroughly comprehend the structure of the eye and be 
familiar with the laws of optics, and yet prove himself a very 
poor optician when it comes to recognizing optical defects and 
-adjusting glasses. 

While education in the abstract is essential, it is, never¬ 
theless, true that no matter how well educated a man may be, 
his education loses half its value if its possessor is unable to 
make practical use of it. This is well illustrated by money, 
which is something we all esteem most highly, which we are 
•constantly striving after, and of which we feel we cannot have 
too much. And yet the value of money lies not in the money 
itself, but in its purchasing power. As long as it is hoarded 
up and hid away it is of no use, but it is only of value for what 
it will buy; hence a man may possess a great deal of money, 
but if, for any reason, he is unable or unwilling to spend it (as 
in the case of the miser), it is valueless. 

While it is all very well for the public to know that the 


PRESBYOPIA. 


365- 


optician has carefully studied his science, yet the main thing, 
after all, is their confidence in his ability to properly fit their 
eyes with glasses when their sight becomes impaired by some 
optical defect. This is his actual business as an optician, and 
it is as such he should strive to be successful. 

A PROFITABLE DIGRESSION. 

The foregoing has been a digression from our subject 
proper—presbyopia—a digression we trust not without profit. 
The consideration of the symptoms of presbyopia, with its 
accompanying eye-strain and other evidences of asthenopia, 
and the necessity for their correction by glasses, if due to pres¬ 
byopia, and the question whether a similar train of symptoms 
might not indicate the possible approach of some serious dis¬ 
ease of the eye incident to the presbyopic period of life, led us 
to a somewhat detailed description of glaucoma, the disease 
most to be feared, and the one most frequently met with, 
which cannot fail to be of benefit to those who have carefully 
read it. 

The danger to the patient’s eve-sight, if this disease was 
not recognized, or if it was confounded with the physiological 
change of presbyopia, and the disastrous effect such an error 
would have on the reputation of the optician who' made it (if, 
indeed, it did not render him liable to a suit for malpractice), 
opened up the much-discussed and never-to-be-settled ques¬ 
tion of oculist versus optician, and just where the province of 
the one ends and the other begins. Feeling that no apology 
is needed for this digression, we pass on now to the 

TREATMENT OF PRESBYOPIA. 

The essential principle of presbyopia being a loss of con¬ 
vexity of the crystalline lens, and a diminution in the refractive 
power of the eye, what more natural than to supply a convex 
lens before the eye to supplement the diminished convexity of 
its dioptric system? Therefore, the treatment of presbyopia 
resolves itself into the adaptation of the proper convex spher¬ 
ical lenses. 

The correction of presbyopia by convex lenses is not quite 
“as old as the hills,” but it dates back some six hundred 


366 


PRESBYOPIA. 


years, and probably even longer, and, therefore, to us who are 
accustomed to the rapid changes and improvements of the 
culmination of the nineteenth century, it seems so long as to 
have always existed. The betterment of the vision of old age 
and the relief of presbyopia called for some remedy, and our 
ancient forefathers proved themselves equal to the task of 
supplying it. 

THE PAST CONTRASTED WITH THE PRESENT. 

How rude and clumsy and imperfect those old-time 
glasses are when compared with the neat and elegant" specta¬ 
cles and eye-glass furnished by the optician of to-day! But 
they served their purpose well, and probably afforded more 
satisfaction to the simple needs of the scholars of that day than 
does a pair of our present faultless eye-glasses to the fastidious 
tastes of the men of our day. Then they were considered a 
luxury, and received the most careful attention, in strong con¬ 
trast to the careless manner and thoughtlessness with which 
they are treated at the present time. 

Notwithstanding the improvement in the grinding of 
lenses and the manufacture of frames, and their better adapta¬ 
tion to the face of the wearer, there has been no change in the 
principle on which they are prescribed, and the convex lens of 
the thirteenth century acted in the same way in relieving and 
assisting the vision of old age as does our much-advertised 
glass of the nineteenth century. There has been marked ad¬ 
vance in the science of optics in the past quarter of a century, 
notably in the recognition and correction of astigmatism, and 
the detection and treatment of muscular anomalies, not to 
mention the more scientific management of hypermetropia and 
myopia; but in the case of presbyopia we do not seem to be 
much ahead of our old-time predecessors. 

THE CORRECTION OF PRESBYOPIA IS NOT A NEW DISCOVERY. 

And, in fact, this is a condition in the treatment of which 
there does not seem to be much room for improvement or ad¬ 
vance. As explained in the early part of this chapter, it is not 
an optical defect, nor /is it to be considered in any sense an 
abnormal process; but it is simply a natural change, a physio- 


PRESBYOPIA. 


367 


logical condition that is common to all mankind, and consists 
in an impairment of the power of accommodation and an ina¬ 
bility to change the focus of the eye so as to adapt it for the 
divergent rays proceeding from near objects. 

The problem to be solved, and the result to be attained, is 
to add, in an artificial way, sufficient refractive power to the 
failing accommodation to maintain the clearness and comfort 
of close vision. 

The sum and substance of the correction of presbyopia is 
the addition of artificial convexity (in the shape of a convex 
lens on the outside of the eye), to supplement the diminished 
convexity of the crystalline lens, and this is the reason that 
the correction of presbyopia is such a simple matter, and ex¬ 
plains why no advancement has been possible over the first 
correction of six hundred years ago. 

THE PRINCIPLE ON WHICH THE CORRECTION OF PRESBYOPIA 
DEPENDS. 

The main symptom of presbyopia, and the external evi¬ 
dence of the senile changes taking place in the eye, is a reces¬ 
sion of the near point beyond a comfortable and convenient 
distance. A gradually receding near point applies to every 
age of life; it is not an accompaniment of age alone, since it 
commences as early as the tenth year; but it is only when it 
has reached an inconvenient distance, which is usually about 
middle age, that it begins to be noticeable and cause its own 
peculiar symptoms, and calls for relief. 

Therefore, when the near point has receded beyond eight 
inches, it has reached a point when it is beginning to blur the 
sharpness of vision for near objects, and the individual feels 
the need of some assistance to clear up the vision, and then we 
look upon the case as one of presbyopia. 

RECESSION OF THE NEAR POINT THE ESSENTIAL FEATURE. 

Such being the case, and the recession of the near point 
being the essential feature of presbyopia, the principle on 
which its correction depends is to furnish a convex lens that 
will bring back this receded near point to eight inches, which 
distance experience has shown to be a convenient one for 


3 68 


PRESBYOPIA. 


reading and writing, and the limit beyond which the eyes can 
be used only with effort and strain. 

We can scarcely imagine the condition of mankind with¬ 
out convex glasses, nor the effect produced on the march of 
progress by the possible withdrawal of presbyopic lenses. 
Without them, persons, as they reach middle life, would be 
compelled to abandon occupations that require sharp vision 
for small objects, and desirable positions that had been accept¬ 
ably filled for so many years would, of necessity, have to be 
relinquished. In order to be able to continue reading, books 
and newspapers would have to be printed in larger letters, and 
the size of the type would have to be graded according to the 
degree of the presbyopia. 

Imagine a customer going into a book-store and inquir¬ 
ing for a book suited to the vision of a man fifty years of 
age, or a newsboy crying out his papers as being printed for 
gentlemen sixty or seventy years of age, or papers of types to 
suit the ages of all customers. 

THE NATURAL FAILURE OF VISION. 

The eye is the most useful, as it is the most wonderful, of 
all our organs of special sense, and the sense of sight differs 
materially from all the other special senses, as does the eye 
differ from all the other organs of the body. In spite of the 
marvelous mechanism of the eye (or, perhaps,'• because of it), 
and although it is such a valuable possession of the human 
race, it is the only organ in the whole range of the human 
system that naturally fails in functional strength and requires 
artificial assistance. 

The more one dwells on this point the stranger does it 
seem, and although the God of Nature surely had some wise 
purpose in view in causing the eye to be subject to these senile 
changes, He has seen fit to withhold the reason from our 
perception. It is certainly not because the eye is in continu¬ 
ous use or that it does not enjoy intervals of rest and repose, 
for the fact is, that sleep affords the eye an opportunity for 
complete rest, and hence the eye is quiescent for seven or eight 
hours out of every twenty-four. In the case of the presbyope, 
where the difficulty of near vision is just beginning to manifest 


PRESBYOPIA. 


369 


itself, at about the age of forty-five, if the individual has re¬ 
ceived his normal allowance of sleep, the eye has been closed 
for fifteen years of this time. 

This is in marked contrast with the heart, which is never 
quiet, but contracts and dilates seventy times in every minute 
every hour of the day, every day of the year, and every year of 
our life, day and night, asleep or awake, from the cradle to the 
grave, and without ceasing its work for a single minute, even 
though the individual reaches a patriarchal age. 

One could hardly wonder if this faithful organ would oc¬ 
casionally ask for a moment’s repose, or if it would require the 
crutch of artificial assistance after years of sleepless service. 
But an all-wise Creator has seen fit to endow the heart with 
such tireless fibre and vigor that there is no natural abatement 
of its force, and only disease and death can still it. 

CONVEX LENSES A BOON TO THE AGED. 

Such being the case, and the function of sight depreciat¬ 
ing so much with the advance of years as to become useless for 
near vision, what a priceless boon the individual finds in con¬ 
vex spectacles properly adapted! They completely neutralize 
and overcome the senile defects in the eye, and restore vision 
to its normal clearness for small objects, and place the presby¬ 
opic individual on the same plane, as regards near vision, as 
his neighbor twenty years younger. By a change of lenses 
for stronger ones, from time to time, as the degree of presby¬ 
opia increases with the addition of years, the near vision is 
maintained sufficiently clear for all practical purposes until a 
very advanced age, unless some diseased condition intervenes. 

DIAGNOSIS OF PRESBYOPIA. 

Ordinarily, if the presbyopia is uncomplicated, it can be 
easily recognized, and is not likely to be confounded with any 
other defect. We say “with any other defect,” but we do not 
look upon presbyopia as a defect; it is a physiological change, 
a natural failure of sight, and should not be classed among the 
defects to which the eye is subject. 

In emmetropic eyes presbyopia makes its appearance 
soon after the fortieth year, at which time the patient seeks a 


370 


PRESBYOPIA. 


better illumination, prefers a larger type, and begins to hold 
his book farther off. The distant vision is unaffected, and in 
every other way the eye is normal. There is no difficulty in 
recognizing this as a case of commencing presbyopia. 

The diagnosis of presbyopia depends on three factors: 

1. Distant vision is perfect; or, in other words, the refrac¬ 
tion is normal. 

2. Near vision is indistinct, and reading type must be held 
farther and farther from the eyes. 

3. The age of the patient: Presbyopia never occurs in an 
emmetropic eye under the age of forty. 

When these three conditions are found in any case, there 
can be no mistake in classing it as presbyopia; and unless all 
three of these conditions are present, it cannot be a case of 
presbyopia. 

The near vision may be impaired by the various optical 
defects, and also by organic disease of some of the structures 
of the eye; but in these cases the distant vision suffers in the 
same proportions, so that the diagnosis seems to hinge on the 
fact that distant vision remains perfect in presbyopia, while it 
is impaired in every other deficiency of sight with which it may 
be confounded. In other words, presbyopia is an error of ac¬ 
commodation (which refers to near vision), and not an error of 
refraction (which refers to distant vision). 

GRADE OF PRESBYOPIA. 

Inasmuch as a recession of the near point is the essential 
feature of presbyopia, the degree of the defect will depend on 
the distance to which the near point has receded, the greater 
the distance of the near point the higher the degree of the 
presbyopia; and it will be measured by the strength of the 
convex lens necessary to restore the near point to a convenient 
distance. 

The near point of distinct vision begins to recede as early 
as the tenth year of life, at which age it is not more than three 
inches from the eyes. This recession is not noticed either by 
the person himself or by his friends, and occasions no incon¬ 
venience in the use of the eyes until it has reached eight inches, 


PRESBYOPIA. 


371 


which usually occurs about the fortieth year of life. At this 
time reading and writing and close work are accomplished 
only at the cost of some strain, and then a weak convex lens 
should be supplied at once, to assist the enfeebled accommoda¬ 
tion, to increase the size of the retinal image, to restore the 
receded near point, and to relieve the strain on the eyes. 

This is the proper principle to be followed in the manage¬ 
ment of presbyopia, to recognize it early, and to supply a weak 
convex lens at once; this tends to preserve arid conserve the 
sight unimpaired the longest possible time, and places the eye 
in the best condition to retard the senile changes. 

It seems, however, as if the majority of people are guided 
by advice just the opposite of this; instead of putting on a 
weak convex lens just as soon as the failure of sight is per¬ 
ceptible, the natural tendency (perhaps in illustration of the 
perverseness of human nature) seems to be to delay the wear¬ 
ing of glasses as long as it can possibly be postponed, to the 
detriment of the eye and the increased impairment of its 
.accommodation. 


PREJUDICE AGAINST GLASSES. 

Many persons are prejudiced against the wearing of 
glasses, and sometimes positively decline to use them, even 
when they are imperatively needed. They may be sensible 
persons, and display good judgment in all other matters, but 
in this one respect they act most foolishly and without any 
reason. A contest with age is hopeless, and it is the part of 
wisdom to yield gracefully to the first summons to surrender. 

PERHAPS IT IS PRIDE. 

Undoubtedly a feeling of pride is one of the chief draw¬ 
backs that tends to make people hesitate about commencing to 
wear glasses, and the fear that it would be an open acknowl¬ 
edgment that they were growing old. This is especially the 
case with ladies, and it seems to be with them an argument 
that is almost unanswerable, and instead of yielding and grow¬ 
ing old gracefully, they endeavor to hide the fact that their 
sight is failing; or, if it becomes evident to others, they make 
all sorts of excuses for it, and try every other means of im- 


372 


PRESBYOPIA. 


proving it that is recommended, except the one proper 
remedy, viz.: glasses. 

This proves itself a matter of contention for the optician,, 
and oftentimes his tact and judgment will be severely strained 
in his efforts to convince people of their simple duty. 

WHAT IS THE NUMBER OF THE FIRST GLASS USUALLY GIVEN 
IN PRESBYOPIA? 

A little thought will show that the number of the glass* 
first prescribed in a case of presbyopia will depend entirely on 
the degree of the impairment of vision present when the 
glasses are first desired. If glasses are sought as soon as the 
symptoms of presbyopia first begin to manifest themselves, a 
+ .25 D. or a + .50 D. lens will usually suffice; but if the 
person refuses to wear glasses, and persists in reading without 
them, and strains his eyes for several years after the need of 
glasses is first felt, then a weak convex lens is no longer suffi¬ 
cient, but a + 1.50 D., or even a + 2 D., may be required; and, 
therefore, if a + .50 D. lens is prescribed in such a case, there 
is disappointment both to optician and patient. 

If the eyes are tested separately in presbyopia, the degree 
of the defect will seem greater, and stronger glasses seem indi¬ 
cated, than when the eyes are tried together. The reason for 
this is, that when both eyes are used and the convergence 
brought into play, the accommodation is stimulated thereby 
and its amount increased, and, therefore, the binocular near 
point (by the use of convergence) is nearer to the eyes than 
the monocular near point (without the employment of con¬ 
vergence); and hence, the accommodation being greater, the- 
degree of presbyopia is lessened. It follows, then, that those 
glasses which would be suited to each eye separately would 
be too strong when the eyes are used together in binocular 
vision. 

THE ASSOCIATION OF ACCOMMODATION AND CONVERGENCE. 

As has been already explained on these pages, there is an 
intimate relation normally existing between the functions of 
accommodation and convergence, which is more or less dis¬ 
turbed by the presbyopic failure of the accommodation. This 


PRESBYOPIA. 


373 


sundering of two functions which have worked hand-in-hand 
for many years cannot occur without producing some disturb¬ 
ance of vision, which will interfere with the comfortable use of 
•the eyes, and will manifest itself by various symptoms and 
indications of irritation. 

In order to preserve as much as possible the harmony be¬ 
tween accommodation and convergence, prisms are sometimes 
employed, combining them with the spherical lenses, so that, 
while the spherical element of the combination assists the 
accommodation, the prismatic element will assist the con¬ 
vergence, provided they are placed over the eye base inward. 

Similar in action to these sphero-prisms are decentered 
lenses. They are ground in the frame in such a manner that 
the wearer looks through a portion of the lens to one or the 
other side of its optical center, and the curved portion of the 
glass that is thus used for vision furnishes a slight prismatic 
action. 

If the student will recall the construction of spherical 
lenses, as described in the earlier chapters of this work, he will 
readily understand how a spherical lens is capable of acting as 
a prism when the rays of light pass through the peripheral 
portions of the lens. 

It will be remembered that a convex spherical lens is 
composed of an indefinite number of prisms, with their bases 
joined together at the optical center of the lens, and a concave 
spherical lens is similarly composed, with the apices of the 
prisms joined together at the optical center of the lens. 

Hence it follows, when it is desired to decenter a convex 
lens so as to assist the convergence, the displacement is in¬ 
ward, and then the prism is presented to the eye with its base 
inward; whereas, in concave lenses, the decentering is outward 
in order to obtain the prism with its base in. 

ORTHOSCOPIC LENSES. 

As a substitute for the regular spherical lenses, and for the 
■sphero-prismatic combination, Dr. Scheffler proposed, some 
years ago, the employment of what he called “orthoiscopic” 
lenses. These lenses are composed of the same two elements, 
a sphere and a prism, but so nicely balanced and proportioned 


374 


PRESBYOPIA. 


that the amount of assistance furnished the accommodation 
and the convergence should exactly correspond. 

As the convex spherical lens removes the necessity for 
any effort of accommodation, and the prism removes the 
necessity for any effort of convergence, an object held at the 
focal distance of the spherical lens may, consequently, be seen 
for an indefinite period of time without any muscular effort 
on the part either of the accommodation or the convergence. 

It will be manifest, on a little reflection, that every convex 
spherical lens must have a corresponding prism which would 
stand in “orthoscopic” relation to it. The following table,, 
which is only approximate, gives the number of the convex 
lens with the degree of the corresponding prism: 


TABLE OF ORTHOSCOPIC LENSES, 


Prisms. 


Lens. 



.50 D. 
I. D. 
r.25 D. 
1.75 D. 
2.25 D. 
3 . D. 


While theoretically correct, the results are not so good as 
at first sight seems possible. One does not have to go very 
high in spherical lenses before the prism becomes so strong, 
and adds so much to the weight and thickness of the glass, as 
to practically prohibit their use, and limits the combinations 
(if employed at all) to the weaker numbers. 

The test of such glasses being perfectly “orthoscopic” is 
that the two lenses, when fixed in their frame, should cast onlv 
a single image upon a card placed at their focal length; this 
naturally calls for careful adjustment. 

Dr. Scheffler’s original proposition was to cut these or¬ 
thoscopic lenses from the peripheral portions of a very large 
lens, in order to obtain the decentering effect of this lens. 

Hence, if these lenses are, or if we consider them to be, 
eccentric portions of one very great lens, we can readily un¬ 
derstand why only one image is formed when they are prop¬ 
erly fixed in their frame and held at their focal distance before 


PRESBYOPIA. 


375 


a screen. A glance at the accompanying diagram will assist 
in making this point clear, as these marked portions, repre¬ 
senting the orthoscopic lenses, are the parts through which the 



Illustration of Orthoscopic Lenses cut from Periphery of a Large Lens. 

eyes would look if the one large lens was held up before the 
face with its center opposite the root of the nose; and, there¬ 
fore, as the large single lens produces only a single image at 
.its focal point, so should its two eccentric portions produce, in 
like manner, a single image at the same point. 

GLASSES SHOULD NOT BE TOO STRONG. 

In old age the convex glasses prescribed for the correc¬ 
tion of the presbyopia should be sufficiently strong to magnify 
the image somewhat, so that it may cover a large nerve sur¬ 
face, and thus impress a greater number of rods and cones, 
and, in this way, partially compensate for the blunted sensi¬ 
bility. 

In the commencement of presbyopia, on the other hand, 
and in the slighter degrees of the deficiency, and while the 
patient retains the normal vigor of middle life, the glasses pre¬ 
scribed should not be too strong. In these cases the thought 
should be kept constantly in mind that the object of the con¬ 
vex glasses is not so much to magnify the retinal images, as to 










376 


PRESBYOPIA. 


bring the near point back to eight inches and restore the point 
of distinct vision to a comfortable and convenient distance 
from the eyes, and, at the same time, to make objects appear 
legible, and as nearly as possible of their natural size, or of the 
size they were before the eyes became presbyopic. 

As previously explained, convex lenses simply add to the 
refractive power of the eye, and supply the loss in the power 
of accommodation incident to age; and when this loss is made 
good, the object for which the glasses are prescribed is fully 
accomplished. Any attempt to do more than this, by the use 
of stronger glasses, may lead to the production of injurious 
results. 

While there are differences of opinion on this subject, 
many gentlemen of experience agree that their observations 
lead them to believe that numberless presbyopic persons, in 
wearing convex glasses, have seriously injured their eyes by 
the use of glasses too strong at first; and thus arose the ne¬ 
cessity for changing them sooner and oftener for those of 
stronger power. 

WHY STRONG GLASSES ARE APT TO BE HARMFUL. 

The statement has been made that the use of strong 
glasses in the commencement of presbyopia is one of the 
factors involved in favoring and increasing more rapidly the 
customary senile changes in the crystalline lens and muscle of 
accommodation. The (Stronger the glass employed, the tess the 
need of accommodative effort, and hence the ciliary muscle 
becomes relaxed from disuse, and even when it is called upon 
to contract its fibres and perform its function, it is only to a 
limited extent. 

A continuance of these conditions results in an enfeeble- 
ment of the muscle of accommodation, which, as a conse¬ 
quence, loses its power to act beyond its accustomed point. 
This point soon becomes so fixed as to indicate the maximum 
tension of the muscle, beyond which it is impossible to go, and 
which point even cannot be long maintained. This requires 
the substitution, in a very short time, of lenses of a much 
higher power, in order to relieve the overburdened accommo¬ 
dation. 


PRESBYOPIA. 


377 


The same principle applies to the use of any other muscle 
in the body; the less it is used and the more it is assisted, the 
sooner it loses its vigor and tone. Hence, it follows that a 
normal use of any muscle or organ of the body is requisite for 
its maintenance in health and strength. 

It should always be remembered that close vision is ac¬ 
complished only by muscular effort, just the same as a volun¬ 
tary act of any other portion of the body. 

The employment of very strong convex glasses in the 
early stages of presbyopia not only weakens the muscle of ac¬ 
commodation by overassisting it, and thus allowing its fibres 
to become relaxed from disuse, but also hastens the senile 
changes in the crystalline lens, and favors the natural tendency 
for an increase in its firmness and solidity. 

In youth, the crystalline lens is soft and yielding, and 
quickly responds in a change of shape at the slightest com¬ 
mand of the ciliary muscle. Near vision calls for an increase 
in its convexity, and more distant vision allows a return to a 
flatter condition. During our waking hours there is, there¬ 
fore, a constant shifting of the particles of the lens substance 
on each other, as the jelly-like body is modified in shape by the 
action of the muscle of accommodation. 

As age creeps on, the lens begins to lose some of its 
original elasticity and softness, and as it grows firmer and 
harder an extra amount of muscular force is now required to 
produce the same degree of convexity of its surface. If, at 
this time, a strong convex lens be supplied, there will be very 
little need for muscular effort, and very little call for a change 
in the shape of lens. Therefore, as the motions of the particles 
of the lens substance on each other are less and less called for, 
the lens begins to lose its elasticity and becomes firmer, and 
offers increased resistance to the action of the ciliary muscle, 
which also is losing its original strength. 

Both of these factors, acting and reacting on each other, 
favor an increase of the essential conditions of presbyopia, 
intensified, as stated above, by the application of strong con¬ 
vex glasses, which weaken the muscle of accommodation by 
removing the necessity for its exercise, and also favor an 
increase in the hardness of the lens by lessening the motion of 


378 


PRESBYOPIA. 


its particles on each other, both of which factors react on the 
ciliary muscle by requiring an increased action from its 
already enfeebled fibres. 

TESTING THE NEAR POINT. 

In the commencement of presbyopia, if the test be made 
in good daylight, the patient may still be able to read ordinary 
print as close as eight inches, and the conclusion might be 
jumped at that presbyopia has not yet set in. But it should be 
remembered that the recession of the near point, and the othei 
symptoms of presbyopia, first manifest themselves in the even¬ 
ing and by artificial light, and if the test be made at the same 
time and under the same conditions, the near point will be 
found to be farther away than eight inches, and, perhaps, nine 
or ten inches will be the closest point at which the reading 
type can be distinguished; and then the existence of pres¬ 
byopia is at once recognized. 

RULE FOR THE SELECTION OF GLASSES. 

The number of the glass required to restore the receded 
near point to eight inches, or, in other words, the lens called 
for to correct any given case of presbyopia, may be obtained 
according to the following rule: 

Subtract the glass which represents the receded near point 
from the glass zvhose focus represents the point zvhich we wish to 
make the near point. 

In making use of this rule, and working it out according 
to the metrical system, the beginner may have some slight 
difficulty at first; but a little practice will enable him to do it 
easily and quickly. 

It should be borne in mind that a meter is equal to about 
forty inches (i m. = 40 in.), and that a centimeter (written 
cm.) is equal to one-hundreth of forty inches, or § of an 
inch (1 cm. = § in.). It should also be remembered, as has 
already been described on these pages, that if the near point, 
expressed in centimeters, be divided into one hundred, the 
result will be the number of dioptres, which, in emmetropia, 
expresses the positive refracting power. 


PRESBYOPIA. 


379 


With these points well understood and constantly borne 
in mind, the optician is prepared to make use of the rule, and, 
in order to assist him, we will give several practical examples 
of it. 

ACCORDING TO THE METRICAL SYSTEM. 

Suppose he meets with a case in which the near point has 
receded to twenty inches. The first step is to reduce the 
inches to centimeters. Forty inches equal one hundred cen¬ 
timeters; twenty inches equal one-half of that, or fifty cen¬ 
timeters (20 in. =• 50 cm.). We then divide this into one 
hundred in order to obtain how many dioptres of positive re¬ 
fracting power it represents (100 -r- 50 = 2), which shows 2 D. 
as the glass representing the receded near point, worked out 
according to the above directions. 

Now, as previously stated in the early part of this chapter, 
we define presbyopia as that condition of the eye in which the 
near point has receded beyond eight inches, and the treatment 
of presbyopia hinges on the restoration of the near point, and 
the bringing it back to eight inches, which is the point we 
wish to make the near point. 

If forty inches equal one hundred centimeters, eight 
inches (being one-fifth of forty) will equal one-fifth of one 
hundred, or twenty (8 in. = 20 cm.). This, then, is to be di¬ 
vided into one hundred, in order to ascertain the number of 
dioptres of positive refracting power; twenty into one hundred 
equals five (100 -r- 20 = 5), indicating 5 D. as the glass repre¬ 
senting the normal .near point. 

Now, to repeat the rule, we subtract the glass represent¬ 
ing the receded near point from the glass representing the 
near point, as we wish to make it, which, in this particular 
case, would be 2 D. taken from 5 D., leaving + 3 D. as the 
glass required to restore the near point to eight inches and 
correct this case of presbyopia. 

By way of further illustration, we will take another case, 
in which the near point has receded to thirteen inches, which is 
represented by a glass of 3 D., according to the following cal¬ 
culation: 40 in. = 100 cm.; 13 in. = 33 cm.; 100 -f* 33 = 3* 
Then 3 D. is to be subtracted from 5 D., which leaves + 2 D. 
as the correcting lens for this imaginary patient. 


- 3^0 


PRESBYOPIA. 


ACCORDING TO THE INCH SYSTEM. 

If the optician wishes to verify the results obtained as 
above, or if he is old fogyish enough to stick to the ancient 
inch system, and refuses to use the (new fangled) metrical 
system, the calculations can be made with fractions somewhat 
as follows: 

We will take the illustrative case first mentioned above, in 
which the near point has receded to twenty inches, which is 
now to be subtracted from eight inches. 

i_i _ 1 ,. 

¥ to — \zVz- 

This involves a troublesome calculation in fractions, and 
illustrates most pointedly one of the chief objections to the 
inch system of numbering lenses. To some of us, who have 
keen out of school for many years, and but little accustomed 
to manipulating fractions, a sum like this would be quite ap¬ 
palling. And to any one it means a sum more or less difficult 
of calculation, as the two fractions must be reduced to a 
common denominator, and then subtracted, and, finally, the 
result must be reduced or changed to its simplest form. 

In the second illustrative case, in which the near point has 
receded to thirteen inches, we have to subtract from eight 
inches, which gives us the following nice little sum in mental 
arithmetic: 

¥ T¥ — TF t- 

The optician who is acquainted with the rule for convert¬ 
ing inches into dioptres, and vice versa, will see at once that 
the results in both cases correspond; in the first case the 3 D. 
lens is equivalent to a thirteen-inch lens, and in the second 
case the 2 D. lens is equivalent to a twenty-inch lens. The 
principle is the same and the result is identical, whether the 
inch or the metrical system be employed; but the latter is 
much to be preferred, for many reasons. 

LIMITATION OF THE RULE. 

Although a rule of this kind is very useful, and glasses 
can frequently be ordered by it with tolerable accuracy, yet it 
has its limitations, and the optician should not expect to 


PRESBYOPIA. 


38 r 

closely follow it in every case that comes to him to be fitted. 
It is always well to bear in mind that the definition we have- 
given of presbyopia, with reference to the recession of its near 
point, is entirely an arbitrary one, and that the optician should 
take into account the distance at which the individual has been 
accustomed to read, or at which he is required to work, and in 
this particular matter there is great variety. 

Many small people work and read at eight inches,, 
whereas very tall people may be uncomfortable unless the 
book they are reading is held fourteen or fifteen inches away.. 
The distance for which the presbyope will require glasses also 
varies according to the occupation in which the person is 
engaged. 

WHAT GLASS TO PRESCRIBE FIRST. 

In the great majority of cases, + 1 D. will be the proper- 
glass to prescribe in the commencement of presbyopia. It 
should be noted that this has reference only to the beginning 
of presbyopia, for it seems to be one of the failings of presby- 
opes that they will defer the wearing of glasses until the latest 
possible moment, many of them being thus actuated by a 
desire to conceal their age and retain their juvenility. 

If, then, a person struggles along for several years after- 
presbyopia has set in, by saving his eyes and surreptitiously 
using another’s glasses, and does not come to the optician 
until his near point has receded to arm’s length, then a + I D. 
will not suffice, but a much stronger glass is required. 

In some cases it may, perhaps, be advisable to commence 
with a + .50 D. lens, if the case is seen at the very outset of 
the presbyopic symptoms. When, as will happen after awhile, 
on account of the steady decline of the accommodation, still 
more power is required, the glasses may be strengthened by a 
half dioptre, as occasion requires; the stronger ones being 
especially needed when artificial light is used, as the symptoms 
of presbyopia are always magnified at night. 

Remarkable evidence of the apparent harmlessness of 
continuous working by the aid of a single strong convex glass- 
is furnished by watchmakers, among whom such work, under- 
such circumstances, is an unavoidable condition of their call¬ 
ing, and who do not appear to be any more liable to eye dis- 


3&2 


PRESBYOPIA. 


eases than others not so engaged; in fact, statistics would seem 
to prove that the habitual exercise of the eye upon fine work 
tends to its development and to the preservation of its powers. 

DIFFERENT GLASSES FOR DAY AND NIGHT USE. 

The glasses first prescribed are usually worn only at 
night, because the need for them is felt principally by artificial 
light. These suffice for a year or two, and then gradually the 
feeling grows on the person that the glasses are scarcely 
strong enough, which feeling is verified by a visit to the opti¬ 
cian’s, where a somewhat stronger pair are procured. At this 
time vision for small objects close at hand begins to be a little 
indistinct, even in daytime, and soon the individual finds he is 
unable, at any time, to read without the assistance afforded by 
convex glasses. Under these circumstances, if the former 
night glasses are worn during the day, they render small 
letters clear and easily legible, even though they no longer 
sufficed for night use. 

In the course of a year or two these glasses are scarcely 
sufficient, even for day use, and then they must be laid aside 
as no longer suitable for the person for whom they were pre¬ 
scribed, in the same manner as a suit of clothes is cast aside by 
the youth who has outgrown them. Then the night glasses 
are again brought into use during the day, and a new and 
stronger pair purchased for night use. These changes should 
not be made until the actual need for them is felt; and the fact 
is, that people generally err on the side of deferring the pur¬ 
chase of new and stronger glasses to the latest possible 
moment, instead of procuring them before the necessity for 
them is experienced. 

GLASSES IN PRESBYOPIA. 

The rule for presbyopia is that two pairs of glasses are 
called for—the stronger pair for night use and the weaker pair 
for day; and when the first become insufficient for their pur¬ 
pose, they are substituted for the day glasses, and a still 
stronger pair procured for use by artificial light. This is a 
very sensible rule, that looks well on paper, and sounds well 
theoretically; but, practically, the optician will have much 


PRESBYOPIA. 383 

difficulty in attempting to persuade his patients to follow its 
teachings. 

There seem to be two chief reasons why this rule has not 
been generally adopted. In the first place, very few persons 
are willing to be bothered with two pairs of glasses, or, if they 
are willing to try it, they never have the right glasses at the 
right time. When they sit down in the evening by the fireside 
to read or sew, they can find only their day glasses, while the 
night glasses cannot be found, and nobody in the family 
knows what has become of them. And, similarly, when they 
want to use their eyes during the day, the night glasses are 
at hand, while the whereabouts of the day glasses cannot be 
discovered. 

In the second place, and after all, since the use of gas 
(especially in the form of the Welsbach light) and electricity 
has become so common, the artificial light is so satisfactory 
that the need for stronger glasses is not felt nearly so much as 
in times past, when tallow candles were the chief source of 
artificial illumination. 

DONDERS’ ADVICE. 

Our great master in accommodation and refraction says: 
“In general it should be observed that it is desirable to ascend 
but slowly in numbers; to use the first spectacles in the begin¬ 
ning only in the evening, and to keep these for day spectacles 
so soon as stronger glasses are required for the evening, and 
thus, every time that the stronger glasses are required, to con¬ 
tinue using the former evening spectacles as day spectacles; 
finally, that while stronger glasses are necessary for reading, 
the weaker are often sufficient for writing and are to be pre¬ 
ferred, since the person wearing them, being enabled to see at 
a greater distance, can avoid the bent position, which is so in¬ 
jurious to the eyes.” 

GLASSES PRESCRIBED ACCORDING TO AGE. 

There has always been a popular notion among the laity, 
and it is still more or less prevalent, that glasses for the correc¬ 
tion of presbyopia can be chosen according to the age of the 
individual, and that for each year of the presbyopic period 
there is a corresponding strength of glass. And even the old- 


384 


PRESBYOPIA. 


time opticians shared in this feeling, and usually made the 
attempt (though oftentimes unsuccessfully) to prescribe the 
convex glasses on the basis of the patient’s age, and without 
making any further inquiries. 

Now, if all eyes were of the same refraction, and at the 
same age began to lose in accommodative power in equal pro¬ 
portion, and the circumstances surrounding the use of the eyes 
in every individual were similar, then all eyes would begin to 
be presbyopic at the same age, and the adaptation of convex 
glasses for the correction of presbyopia would be reduced to 
the simple question of asking the age of the patient, and a 
table could readily be prepared that would be a trustworthy 
guide for optician or patient in selecting glasses according to 
the rule referred to, which, under the circumstances, would be 
infallible. 

But no two pairs of eyes seem to be exactly alike in their 
physical condition or in the manner in which they are brought 
into daily use; and hence the individual differences in the state 
of the refraction and the accommodation, at the specified 
periods of life, are too great for the preparation of a rule for 
the prescribing of glasses according to age, that can have any 
real value except approximately. Therefore, this method of 
fitting glasses cannot be relied upon, and should not be re- 
sorted to by the optician in his management of cases of pres¬ 
byopia, to the exclusion of an individual examination in each 
case. 

TABLES FOR PRESBYOPIA. 

In persons with emmetropic eyes, in good bodily health, 
and with no symptoms of premature senility, and if the eyes 
are not subjected to any unusual strain, or used under unfavor¬ 
able conditions, a table may be prepared which will approach 
the results obtained by a skilled examination in each individual 
case. 

A common rule is to commence with + 1 D. at forty-five 
years of age, and add + 1 D. for every five years, as follows: 


Age of Patient 

Glasses Requii 

45 

+ 1 D. 

50 

+ 2 “ 

55 

+ 3 “ 

60 

+ 4 “ 

70 

+ 5 “ 


PRESBYOPIA. 


385 


Another table, more particular and more nearly correct, 
perhaps, has been figured out on the lines laid down by Don- 
ders. This commences at the age of forty-five with a + .50 
lens, which, in most cases, suffices (instead of the + 1 D. called 
for in the above table): 

Age of Patient. 

45 
48 
50 
55 
58 
60 
62 

65 

70 
75 
78 
80 

• While this table is not to be used as a basis on which to 
order the glasses, yet it can be taken as a guide-post pointing 
to the normal condition, and any marked departure therefrom 
would indicate some abnormality of refraction. If much 
stronger glasses are necessary at any particular age than the 
table indicates, hypermetropia is to be suspected. While, if 
the need of glasses is postponed much after the age of forty- 
five, and much weaker glasses suffice at any specified age than 
the table would indicate, myopia is most likely to be present. 

PRESBYOPIA WHEN COMPLICATED. 

It is the proper method, in all cases of presbyopia that 
apply for glasses only for reading, to ascertain the acuteness 
of vision and determine the condition of the refraction of the 
eye. As soon as the optician calls the attention of the patient 
to the card of test-letters hanging across the room, the almost 
invariable reply of the patient is, that their distant vision is all 
right, and that they do not need glasses to see off or across the 
room, but want them only for reading. This necessitates the 
optician telling his patient why the distant vision must be 
tested in order to ascertain the real refractive conditon of the 
eye. 


Glasses Required if 
Emmetropic 
+ .50 D. 

+ .75 “ 

+ 1 “ 

+ 1 50 “ 

+ 2 “ 

+ 2.50 “ 

+ 3 “ 

+ 4 “ 

+ 5-50 “ 

+ 6.50 “ 

+ 7-50 “ 

+ 9 


386 


PRESBYOPIA. 


The cases of presbyopia that will require to be the most 
carefully fitted are those which are complicated with some 
existing error of refraction. Perhaps glasses may have been 
worn for the optical defect for many years, and as the person 
grows older, and gets into middle life, presbyopia begins to 
make its appearance and complicate the defective vision, and 
then the glasses will have to be changed to meet the altered 
conditions. 


HYPERMETROPIA AND PRESBYOPIA. 

As hypermetropia is the predominant errpr of refraction, 
so it will complicate many cases of presbyopia. Hyperme¬ 
tropia in many persons first shows itself as an early pres¬ 
byopia; that is, the individual may not be aware that his eyes 
have any defect in their optical condition, because the hyper¬ 
metropia exists in a latent form; but as age creeps on the 
latent defect gradually becomes manifest, and the condition of 
presbyopia is made to commence much earlier than it other¬ 
wise would in emmetropic eyes. 

The optical student who has understanding^ read the 
preceding pages will appreciate the reason for this; during his 
earlier years the patient was able to overcome the hyperme¬ 
tropia while the lens was soft and elastic, and without any 
appreciable effort; but as time goes on the refraction dimin¬ 
ishes, and the near point recedes with him much earlier in life 
than with thie emmetrope, and he will require a weak convex 
glass for reading long before he is forty years of age. 

If hypermetropia is suspected as existing in any case in 
connection with the presbyopia, the eyes should be tested 
for it. In fact, in the routine examination of any case of 
presbyopia, it is the proper thing to determine the condition 
of the refraction, and to detect any departure from the emme¬ 
tropic state. For, it may be repeated, in presbyopia, pure and 
simple, the refraction is normal and the distant vision is unim¬ 
paired. Just as soon as any error of refraction is detected in 
any supposed case of presbyopia, it ceases to be a case of pres¬ 
byopia, and should be classed with that particular defect that 
is found to be present, which is then complicated with ap¬ 
proaching old sight. 


PRESBYOPIA. 


387 


TESTING FOR HYPERMETROPIA. 

If much stronger glasses are required at any particular 
age than the foregoing tables would indicate, the presence of 
hypermetropia is suspected. The diagnosis of hypermetropia 
depends upon the acceptance of a convex lens for distance. 
The patient is requested to name the letters on the test-card 
hanging twenty feet away; this determines his acuteness of 
vision, which, in some eases, is normal, although it is usually 
below the standard. Then a weak convex lens is placed before 
the eye, followed by stronger and stronger ones, and the 
strongest convex lens that is accepted will be the measure of 
the hypermetropia. 

If the convex lenses improve the clearness of the letters, 
and raise the acuteness of vision, or if they do not blur the ap¬ 
pearance of the letters, but allow them to be read equally as 
well with as without the lenses, the presence of hypermetropia 
is proven. But, on the other hand, if all convex lenses blur 
the letters, and even the weakest is rejected, the optician may 
reasonably conclude that the patient has no hypermetropia, at 
least no manifest hypermetropia. It might be remarked in 
parenthesis that there is but little likelihood of the defect ex¬ 
isting in a latent form, because at the presbyopic age latent 
hypermetropia becomes manifest and is no longer concealed 
by the weakened accommodation. 

CORRECTION OF HYPERMETROPIA AND PRESBYOPIA. 

If hypermetropia is found to be present, it must first be 
corrected by the proper neutralizing glasses, as indicated 
above; these glasses may, perhaps, even suffice for reading for 
a while, but sooner or later, in all cases, additional strength is 
required for reading. 

The answer to the question as to whether the glasses that 
correct the hypermetropia are sufficiently strong for reading, 
will depend on the amount of available accommodation en¬ 
joyed by the patient, with the assistance of the glasses, or, in 
other words, on the position of the near point. If, with the 
glasses, the near point is still within eight inches, nothing 
more is needed for reading for the present. 

But if with the glasses the near point has receded to ten 


388 


PRESBYOPIA. 


or twelve inches, the eyes are in need of additional assistance, 
and the number of the glasses required is obtained by the same 
calculation as in simple presbyopia. 

For instance, if + I D. was the strongest lens accepted for 
distant vision, we would consider that lens as the measure of 
the hypermetropia. With this glass over the eyes the near 
point is measured and found to have receded to ten inches. 
Now, the glass representing the receded near point of ten 
inches is 4 D., and the glass representing the near point as it 
should be (eight inches) is 5 D.; then the calculation is to sub¬ 
tract 4 D. from 5 D., which leaves 1 D. as the measure of the 
presbyopia over and above the hypermetropia. 

This patient, therefore, has 1 D. of hypermetropia, and an 
additional 1 D. of presbyopia. The sum of these two glasses 
combined will give the actual amount of presbyopia, and will 
be the glasses needed for reading. Two pairs of glasses are, 
therefore, required—one for distance and constant wear, and 
another (and stronger) for reading. 

In cases of hypermetropia as slight as this, the acuteness 
of vision is not usually much impaired, and the need of glasses 
is not felt for distant vision. In such a case the + 2 D. glasses 
are prescribed for reading, and the patient gets along very well 
until the next change of glasses is needed, which may not be 
for three or four years. 

If, for any reason (asthenopia perhaps), it seems desirable 
to order glasses for distance, a pair of + 1 D. spectacles are 
given for that purpose, which are substituted by the + 2 D. 
when reading or close work is desired. Or, instead of remov¬ 
ing the + 1 D. distance glasses when he wants to read, he may 
have another pair of + 1 D. lenses in an extra front frame, 

which he hooks over his distance 
glasses, and the sum of the two 
together affords him just the 
strength he needs for reading. 

Or, if the patient doesn’t want 
to go to even this much trouble, 
the glasses may be ordered “bi¬ 
focal,” in which the upper portion 
of the glass is that required for dis- 




PRESBYOPIA. 


389 


tant vision, while the lower segment represents the glass re¬ 
quired for reading. 

“Split glasses” were in use for many years as the best 
form of bifocal glasses obtainable; 
but recently they have been super¬ 
seded by the new form, in which 
the size of the distance portion of 
the glass is increased at the expense 
of the reading part. 

In some cases the reading 
power is obtained by cementing a 
small thin segment on the lower 
part of the distance glass; and in other cases the lower part 
of the distance glass is cut out and replaced by the proper 
reading lens. 

MYOPIA WITH PRESBYOPIA. 

Presbyopia also comes to those who are myopic; but in 
this case the myopia tends to neutralize the presbyopia and 
retards the recession of the near point, and hence the incon¬ 
veniences of presbyopia are not experienced until much later 
in life. In cases of high degrees of myopia, where the far 
point lies at or nearer than eight inches, presbyopia can never 
occur. 

We often hear of persons who have reached fifty-five or 
sixty years of age and are still able to read without glasses, 
and they are spoken of, and pointed out, as persons possessing 
wonderful sight; but the fact is, that the persons who are able 
to read at this age without glasses are usually myopic, and an 
examination of their acuteness of vision will soon determine it. 
If the myopia is not of high degree, they may have gone 
through life without being conscious of it, and are disposed 
to attribute their ability to read without glasses, after middle 
life, to the exceptional strength of their eyes. 

In cases of myopia of 2 D. or 3 D., where, perhaps, the 
glasses have been worn constantly, as the person approaches 
the presbyopic period of life, he finds he can read better 
without his glasses, although he still needs them for distance. 
As he passes on into the fifties, he finds that reading is not so 




390 


PRESBYOPIA. 


pleasant as formerly; in fact, he begins to feel the need of some 
assistance. In this case the myopia, by increasing the refrac¬ 
tion of the eye, had kept the near point within eight inches, but 
now an examination will show that it has receded beyond this 
point. A weak convex lens that will restore the receded near 
point and enable the person to read comfortably at the usual 
distance, will be all that is required to relieve the presbyopic 
symptoms, while the same concave lenses may still be worn 
for distance. 

In the higher grades of myopia (5 D. and over), pres¬ 
byopia, strictly speaking, cannot occur in the sense that the 
near point cannot recede beyond eight inches; but such eyes 
are not excepted from the usual changes that accompany age; 
even in myopia the crystalline lens grows harder and denser, 
and the ciliary muscle (never very strong) grows weaker. 

In youth, when the lens is elastic, the accommodation is 
sufficiently strong to allow the same glasses to be worn con¬ 
stantly for reading as well as for distance; but, on account of 
the inevitable senile changes, when the patient reaches forty 
years of age he is scarcely able to read with his glasses any 
more, nor would it be prudent for him to read without them. 
In this case we want the effect of a convex lens, which we can 
obtain by ordering a weaker concave glass for reading, which 
the patient will be able to use at the proper reading distance 
with comfort. 

When a patient wearing — 8 D. glasses for distance 
reaches fifty years of age, his glasses will make the print so 
small and blurred that he will be unable to read. Now, ac¬ 
cording to the table, an emmetropic eye at this age has a pres¬ 
byopia of 2 D., and if we place a + 2 D. lens before the — 8 D. 
glass, we have — 6 D. as the proper glasses to prescribe for 
this patient for reading. 

But in many cases of myopia the accommodation is so 
feeble that the reading glasses calculated by this rule would be 
too strong; and, perhaps, the necessary glasses can be more 
accurately determined by the following method: We reduce 
the grade of the myopia just so much as to afford a convenient 
far point; for instance, a myopia of 3 D. has a far point of 
thirteen inches, which is a comfortable reading distance. If 


PRESBYOPIA. 


391 


we prescribe — 5 D. glasses, we leave 3 D. of miyopia uncor¬ 
rected; and, therefore, a pair of - 5 D. glasses for reading 
would afford a far point of thirteen inches, and would, proba¬ 
bly, prove more satisfactory than the — 6 D. glasses men¬ 
tioned above. 

The celebrated philosopher, Benjamin Franklin, was my¬ 
opic, but not to a very high degree. He wore concave glasses 
for distance, and after he passed (sixty years of age be wore 
convex glasses for reading. He wore the divided glasses, the 
upper half concave, the lower half convex. For this reason 
such glasses are often called Franklin lenses. 

PRESBYOPIA WITH ASTIGMATISM. 

In those cases of presbyopia where the refraction is com¬ 
plicated by astigmatism, there is sometimes difficulty in mak¬ 
ing a satisfactory adjustment. The rule here is to determine 
the refraction of the eye and correct the astigmatism, and then 
add to these cylindrical glasses suitable convex lenses, such as 
will make the vision clear and 'comfortable at the reading 
point. 

If the astigmatism is slight in degree, it may sometimes 
be disregarded in making the presbyopic correction, and simple 
convex lenses prescribed with entire satisfaction. But if 
marked astigmatism is present it cannot be ignored, but must 
be included in the correction. 

SIMPLE HYPERMETROPIC ASTIGMATISM. 

The astigmatic patient who applies to the optician for 
glasses for reading (presbyopia), may wear cylindrical glasses, 
or, perhaps, may not. If the former, the optician can easily 
measure the glasses to ascertain what they are. If the latter, 
the method of examination which we have taught the optician 
to pursue will reveal the defect, and, at the same time, indicate 
the correcting lenses. 

Suppose a patient comes with simple hypermetropic astig¬ 
matism, and the correcting lens (no matter whether previously 
worn or if just determined by the optician) is 
+ 1.50 D. Cyl., axis 90°. 

In this case the refraction of the two chief meridians is in¬ 
dicated and illustrated by the following figure: 


392 


PRESBYOPIA. 


Emmetropic. 


Hypermetropia 
15.0 D. 


The vertical meridian is emmetropic, and the defective 
horizontal meridian is made iso by the correcting cylinder. 
This makes both meridians emmetropic and places the eye on 
the same plane as an emmetropic eye, and, therefore, subject 
to the same rules for the correction of presbyopia as in emme- 
tropia. 

The patient is forty-eight years of age, and the measure¬ 
ment of the near point (with the correcting cylinders before 
his eyes) shows it has receded to eleven inches. By following 
the rules given in the earlier pages of this chapter for measur¬ 
ing presbyopia and determining the glasses required to relieve 
it, we find this patient needs glasses of + 1.50 D. With these 
glasses placed in the trial-frame over the cylinders, the print is 
made clear and legible and reading becomes pleasant and 
comfortable, and the patient is satisfied this is just what he 
needs for reading. 

The prescription would read as follows: 

+ 1.50 D. S. 3 + 1.50 D. Cyl., axis 90°. 

This is a sphero-cylindrical lens, and must be ground to 
order for this patient. 

SIMPLE MYOPIC ASTIGMATISM. 

Another interesting class of cases, with which presbyopia 
may be complicated, is that of simple myopic astigmatism, in 
which the one meridian is myopic and the other meridian is 
emmetropic. 

Now, if we examine each meridian separately, we will find 
that in the myopic meridian the patient will not need a convex 
glass for reading in the commencement of the presbyopic 




PRESBYOPIA. 


393 


period, because the refraction in that meridian is the same as, 
in a case of simple myopia, and, hence, is adapted for the 
divergent rays coming from a close point. 

But in the meridian at right angles to this, which is em¬ 
metropic, reading is blurred and indistinct, and there is every 
evidence of the existence of presbyopia, and the patient will 
require the same strength of convex lens for reading as the 
wholly emmetropic person of the same age requires. 

Perhaps these points can be made more clear by an illus¬ 
tration. Suppose we have a case of simple myopic astig¬ 
matism of i D., which might be expressed as follows: 

V. = -f§ ; with — i D. Cy 1 -. axis i8o°, V. = f °. 

Myopia 1 D. 


Emmetropia. 


In this case we have a myopia of i D. in the vertical me¬ 
ridian, while the horizontal meridian is emmetropic. Now, a 
person with uncomplicated presbyopia, at this age, would re¬ 
quire about a + i D. glass to enable him to read comfortably 
at the proper distance; and, as our imaginary patient is emme¬ 
tropic in the horizontal meridian, so he would require a con¬ 
vex lens of i D. to correct this meridian; while the vertical 
meridian, being myopic to the extent of I D., would require 
no glass for close vision, as the necessary convexity in this 
meridian is supplied by the myopic astigmatism. 

Hence we have to assist the refractive power of the hori¬ 
zontal meridian alone, which we can do by ordering 

+ i c M axis 90° 

for reading, while 

— 1 C., axis 180 0 

may still be worn for distance. 




394 


PRESBYOPIA. 


The correcting concave cylinders place the eye on the 
same plane as an emmetropic eye, which, at this age, would 
require + I D. for its presbyopia. If, in like manner, we add 
+ i D. to the glasses of this astigmatic presbyope, our pre¬ 
scription would be 

+ I D. S. 3 — I D. Cyl., axis i8o° 

This is practically the same prescription as the above, and the 
order for the glasses may be written either way, as each is 
correct. 


COMPOUND HYPERMETROPIC ASTIGMATISM. 

In cases of compound hypermetropic astigmatism that 
reach the presbyopic age, we follow the same rules of calcula¬ 
tion, and add the presbyopic correction to the original 
sphero-cylinder, always remembering that the greater the de¬ 
gree of hypermetropia, the stronger will be the convex 
spherical element of the combination, modified by the age and 
bodily condition of the patient. 

Hypermetropia 1 D. 


Hypermetro 


Suppose we have a patient who has been wearing the fol¬ 
lowing correction for distance and for constant wear: 

+ I D. S. 3 + I D. Cyl., axis 90° 

as indicated by the above diagram. Such a patient will begin 
to feel the need of glasses, and show the symptoms of presby¬ 
opia, about the age of forty years, when an additional + 1 D. 
will be called for, for reading, and the prescription would be: 

+ 2 D. S. 3 + 1 D. Cyl., axis 90° 




PRESBYOPIA. 


395 


COMPOUND MYOPIC ASTIGMATISM. 

In this condition of refraction, the presbyopic correction 
must be subtracted from the concave spherical element of the 
combination, while the cylindrical element remains un¬ 
changed, and, hence, the greater the degree of myopia, the 
weaker will be the convex glass required, according to the age 
and general physical condition of the patient. The calculation 
in this case may seem more complicated than in the preceding 
case. 

Myopia 4 D. 


Myopia 2 D. 


Suppose we have a patient who has been wearing the fol¬ 
lowing combination for constant use: 

— 2 D. S. 3 — 2D. Cyl., axis l8o°. 

When this patient reaches forty years of age, these glasses will 
not seem so pleasant and satisfactory as formerly. This is due 
to the approach of the presbyopic changes, and the patient 
must have the benefit of a convex lens of about I D. We add 
it to the combination with the following result: 

— 2 D. S. 3 — 2 D. Cyl., axis i8o° 

+ ID.3. 

— i D. 8. 3 — 2 D. Cyl., axis i 8 o°. 

These glasses will be a great improvement for a time; but 
after awhile they will no longer suffice, and the patient will 
need the effect of a + 2 D. lens; and then we have the follow¬ 
ing sum to work out: 

— 2 D. S. 3 — 2D. Cyl., axis i8o° 

+ 2 D. S. 

— 2 D. Cyl., axis i8o°. 

Now the correcting lens for reading is a plain cylinder. 






I 


39b PRESBYOPIA. 

But the presbyopic changes are constantly progressing, 
and the glasses do not last forever, and, in the course of a few 
years, a lens of + 3 D. would ordinarily be called for, and 
must be added to the combination with the following result: 

— 2 D. S. 3 — 2D. Cyl., axis i8o° 

+ 3 D. S. 

+ 1 D. S. 3 — 2D. Cyl- axis 180 0 . 

And thus the varying changes in the dynamic refraction of the 
eye are met with corresponding changes in the spherical ele¬ 
ment of the correcting combination. 

MIXED ASTIGMATISM. 

Myopia 1 D. 


» Hypermetropia 
1 D. 


Suppose we have a patient with mixed astigmatism, who 
lias been wearing the following cross cylinders: 

+ I D. Cyl., axis 90° I D. Cyl., axis l8o°. 

When this patient reaches forty-five years of age, he will 
complain that the reading is blurred and indistinct; or, per¬ 
haps, he will scarcely be able to read at all. If we place a + I 
D. lens over his spectacles, he says the print is made much 
more distinct, and he can read now with comfort. This gives 
us the following sum: 

+ I D. cyl., axis 90°, 3 — 1 D. Cyl., axis 180°. 

+l D.S- 

But we cannot add a spherical to, nor subtract it from, a 
cylinder; and, hence, the result of this sum is not so easy to 
ascertain. 

When the case is analyzed, however, and the refraction of 
each meridian considered /separately, the problem is much sim¬ 
plified, and may be stated as follows: 






PRESBYOPIA. 


397- 


Horizontal Meridian + i. Vertical Meridian — i. 

_+ _£._ + I. 

Horizontal Meridian + 2. Vertical Meridian o. 

The result of this sum is expressed by the following pre¬ 
scription : 

+ 2 D. Cyl., axis 90°. 

The cross cylinder in this case may be transposed into a 
sphero-cylinder, and written in two different ways: 

+ 1D.S.3 — 2D. Cyl., axis 180 0 
or — 1 D. S. 3 + 2 D. Cyl., axis 90° 

Now, df we add + 1 D. s - to the first combination, the re¬ 
sult is + 2 D. s * 3 — 2D. G f L ’ axis 180°, which can be 
reduced to + 2 D. c y L * axis 90°, because the minus cylinder 
neutralizes the plus spherical in the vertical meridian. 

If we add + I D. s - to the second combination, the -f 1 
will neutralize the — 1 and leave + 2 D. G ? h ’ axis 90°, as the 
result, which is the same in every case, even when the problem 
is differently worked out. 

METHOD OF EXAMINATION. 

The clinical investigation of any case of supposed pres¬ 
byopia should commence, first of all, with a test of the static 
refraction of the eye, and a determination of the acuteness of 
vision. This will reveal the existence of any hypermetropia, 
myopia or astigmatism, and render possible the classification 
of the presbyopia, as to whether it is simple or complicated. 

The methods in common use for the detection of these 
defects have already been described at considerable length in 
the previous chapter, and need not be repeated here. Suffice 
it to say, that in hypermetropia the measure of the defect must 
be added to the value of the glasses ordinarily required by a 
presbyopic emmetrope of the same age, while in myopia the 
degree of defect must be subtracted, in order to arrive at an 
approximate estimate of the glasses required for reading. In 
astigmatism the correcting cylinder must be combined with 
the requisite convex spherical lens. 

Only in patients where the refraction is emmetropic and 
the acuteness of vision measures up to the normal standard, is 
the case to be considered one of simple and uncomplicated 



39§ 


PRESBYOPIA. 


presbyopia, and to be measured and corrected according to the 
rules laid down for this condition. 

TESTING NEAR VISION. 

After the determination of the condition of the refraction 
(the tests for which are made at a distance of twenty feet), and 
not before, a trial should be made of the near vision, to ascer¬ 
tain the reading capacity and to measure the amplitude of ac¬ 
commodation. This will give the position of the receded near 
point, on the distance of which depends the degree of the pres¬ 
byopia; while the principle involved in the correction of the 
disability is to supply a glasis that will restore the receded near 
point to a convenient distance, and supplement the failing 
accommodation. 

This confines the treatment of presbyopia to a palliation 
of the impaired condition of the sight. However, when suita¬ 
ble glasses are prescribed, the individual is enabled again to 
use his eyes freely for near work without fatigue. If the 
glasses are too weak, they fall short of affording the full 
measure of relief; while if they are too strong, they necessitate 
the holding of the book too close to the eyes, and thus impose 
extra work upon the convergence, and may give rise to symp¬ 
toms of asthenopia. 

GLASSES MUST BE CHANGED. 

The need for a change of glasses from time to time will be 
felt in all cases of presbyopia, due to a steady and continued 
loss of accommodation. This need will be accompanied by the 
same symptoms that indicate the commencement of presby¬ 
opia, and it arises at intervals until the accommodation is 
entirely gone, when the patient may not require any further 
change of glasses for many years. 

The frequency of these changes varies much in different 
individuals, depending on the innervation of the eye and the 
sensitiveness of the patient to slight inconveniences, as well as 
the nature of one’s occupation, and the degree and accuracy of 
sight required. In general, they should be made as often as 
once in every two or three years; not oftener than every two 
years without exciting suspicion of the existence of some com- 


PRESBYOPIA. 


399 


plication that endangers vision; nor longer than three years, 
else the eyes will be strained by reason of the glasses being of 
insufficient power. 

The amount of change, or the difference between the old 
and the new lenses, will vary with the interval that has elapsed, 
and the rapidity of the failure of the accommodation. Each 
time such a change is to be made the new lenses must be 
chosen according to the same rules that determined the choice 
of the old ones, or according to the rules laid down for com¬ 
mencing presbyopia. 

5 D. OF ACCOMMODATION NECESSARY FOR NEAR VISION. 

The amount of available accommodation should not fall 
below 5 D., to the end that near vision may be pleasant and 
comfortable. As presbyopia steals on, and it begins to fall 
below this point, we supply the deficiency by placing a convex 
lens on the outside of the eye. Thus the accommodation 
steadily decreases and the convex lens as steadily increases, 
until, finally, there is an entire loss of accommodation, and we 
find a 5 D. lens supplying the necessary reading power. 

This lens would always suffice thereafter if the dioptric 
system of the eye remained stationary. But the senile changes 
do not stop with an entire loss of accommodative power, but 
continue until they cause the eye to pass over into a condition 
of acquired hypermetropia, when the 5 D. lens will no longer 
suffice. Now a glass is called for, not only to take the place of 
the lost accommodation, but also to correct the supervening 
error of refraction. 


AMBLYOPIA. 

Amblyopia (which is an impairment of vision not due to 
refractive errors which can be corrected by glasses, but de¬ 
pendent upon organic disease which places it beyond the op¬ 
tician’s help) sometimes exists in connection with presbyopia, 
and may even be mistaken for it, because the amblyopic 
patient, in like manner, cannot see very small objects dis¬ 
tinctly, and sometimes, also, convex glasses improve his 
vision. But in simple presbyopia (uncomplicated with ambly¬ 
opia) the patient enjoys the normal acuteness of vision and an 


400 


PRESBYOPIA. 


unrestricted range of accommodation, which would be impos¬ 
sible in the presence of amblyopia Besides, with the proper 
convex glasses the patient is able to read the No. I type as 
close as eight inches; but if he can read only the No. 3 or 
No. 4, and that with a conscious effort, or is obliged to hold 
the book at some unusual distance, we may reasonably infer 
that he is amblyopic. 

GLASSES SHOULD NOT MAGNIFY TOO MUCH. 

It should always be borne in mind that the object of the 
glasses prescribed for the relief of presbyopia is not to magnify 
the print, this being merely an incidental effect, but rather to 
add to the refractive power of the eye and assist the crystalline 
lens in focusing divergent rays of light upon the retina. Only 
so much assistance should be given as is really required, and it 
follows that anything more than this would be not only super¬ 
fluous, but injurious. 

The magnifying of the print produced by the convex 
glasses worn by the presbyope depends on two factors: First, 
the enlarging effect of the convex lens itself (this power being 
inherent in all convex lenses); and, secondly, the contrast with 
the appearance of the letters before the glasses were used, the 
print for some time previously having been diminishing in size, 
on account of the lessened refractive power being scarcely 
sufficient to bring it to a focus on the retina. Hence, when 
glasses are worn and the refraction increased, a clear and dis¬ 
tinct image is formed, which contrasts strongly with the pre¬ 
vious indistinct one. 

PRESCRIBING GLASSES FOR PRESBYOPIA SHOULD NOT BE 
CARELESSLY DONE. 

Although it seems to be a very simple and easy matter to 
adjust glasses for presbyopia, the truth is, mistakes are not 
uncommon, more so, perhaps, than in the correction of some 
of the other errors of refraction. If such is the case, the cause 
is to be found in the fact that presbyopia is not a well-defined 
departure from the normal form or structure (as are the 
various optical defects), but is rather an impairment of func¬ 
tion of the crystalline lens and the ciliary muscle, the latter of 


PRESBYOPIA. 


4 OI 

which does not always enjoy the same degree' of innervation, 
varying greatly at different times and under differing circum¬ 
stances. 

In youth the muscle possesses a normal tone, which is 
constant, as a rule; but there is a wide departure from this 
condition in presbyopia, 

A patient who is apparently satisfied and pleased with his 
glasses one ciay, may return the next day with the complaint 
that they are entirely unsuitable. This is, perhaps, more often 
the case with women, because they do not seem to be able to 
interpret their sensations and express their feelings as accu¬ 
rately as men. 1 ^ j 

GIVE THE PATIENT A CHANCE TO EXPRESS HIS CHOICE. 

The optician should not follow too closely any iron-bound 
rule in prescribing glasses for presbyopia. Sometimes a 
stronger glass, and sometimes a weaker one, will give more 
satisfaction than the one that seems to be indicated. 

The effort should always be made to afford the patient as 
extended a range of accommodation as possible, by giving the 
glasses that allow the greatest distance between the near point 
and far point of distinct reading vision. This will operate in 
the direction of forbidding a glass that is too strong, because 
the stronger the glass the more restricted will be the range of 
accommodation. 

This calls for the trial of several pairs of glasses, those 
slightly stronger and those a little weaker, in connection with 
the pair that seems ito be indicated by the test that has been 
made. Then the patient should be allowed to express his 
preference for the pair of glasses that seems to him to be the 
most satisfactory. Not that the patient’s choice should neces¬ 
sarily determine the matter, but because his preference should 
be given some consideration in arriving at a final decision. 

COMMON SENSE TO BE USED. 

Sometimes the patient may be right and sometimes he 
may be wrong, but the optician, in the light of the knowledge 
gained by his examination, will be able to determine which it 
is. If your judgment sustains the patient’s choice, it is a 


402 


PRESBYOPIA. 


satisfaction to him to receive the glasses which he feels are 
suitable. But if otherwise, you must be the final arbiter, and 
the responsibility for the proper selection rests with you. We 
occasionally meet with people who cannot be made to say that 
they are satisfied, either because of natural perverseness, or 
because the case is not an uncomplicated one. 

Even though our scientific tests indicate a certain number 
of glass, we must not too strongly force it upon a patient 
against his own judgment. There must be an admixture of 
common sense with science, if the prescribed glasses are to be 
a success, and if the scientific optician wishes to avoid ship¬ 
wreck of his superior skill and knowledge. After trying 
several pairs of glasses, the patient sometimes becomes con¬ 
fused, or bis eyes get "tired, and it is impossible for him to tell 
which pair suits him best; then the optician dispels the clouds 
and prescribes the glasses indicated by his tests. 

PURPOSE FOR WHICH THE GLASSES ARE INTENDED. 

The question should always be asked for what purpose 
the glasses are intended; are they desired for reading alone, for 
reading and writing, or also for sewing and working, and, in 
the latter case, at what distance the work is performed, as the 
strength of the glasses will vary somewhat with the use to 
which they are to be placed. 

FITTING THE FRAME. 

Great care should be taken in fitting the frame for reading 
glasses, to see that they are correctly centered and properly 
placed before the eyes, so that the eyes may look through the 
centers of the glasses, as otherwise the prismatic effect of the 
lens is unintentionally called into action. 



The normal position for glasses is when they are correctly 
fitted over eyes that are looking at distant objects, the visual 
axes being parallel and corresponding exactly with the optical 


PRESBYOPIA. 


403 


centers of the lenses, and they, in turn, with the geometrical 
centers. For reading, the optical centers may be a trifle 
closer, a little lower, and the plane of the glasses may be in¬ 
clined forward, so as to maintain it as nearly as possible at 
right angles to the line of vision. 

A convex lens is thickest at the center, and may be looked 
upon as composed of an indefinite number of prisms with their 
bases joined at the optical center. If the line of vision pass 
through this point, it is unaffected; but if it pass to either side, 
a prismatic effect becomes noticeable, the more marked the 
farther from the center the line passes. 



If the frame is too wide, the pupils will look through the 
inner edge of the lenses, which will then act as prisms with 
their bases outward. This increases the need for convergence, 
taxes the internal recti muscles, and strains the eyes. If the 
frames are too narrow, the line of vision will be directly 
through the outer edges of the lenses, with the effect produced 



of prisms with their bases inward. This assists the conver¬ 
gence and relieves the internal recti muscles; but, at the same 
time, disturbs the harmony that naturally exists between the 
accommodation and convergence, and in this way may be pro¬ 
ductive of symptoms of asthenopia. In case the frame should 
be improperly fitted, there would be much less discomfort to 
the eyes from frames that are too narrow than from frames 
that are too wide. 

Oftentimes, when patients complain that their glasses are 
not satisfactory, the cause of the trouble may be found in the 
improper adjustment of the frame. This is an important 
matter, and if neglected may destroy the benefit of the most 
carefully adjusted lenses. 


404 


PRESBYOPIA. 


ORTHOSCOPIC LENSES. 

In regard to the use of orthoscopic lenses (which have 
been described in this chapter) for the relief of presbyopia, the 
majority of persons wearing reading glasses will get along 
well enough without them. In the great number of cases, 
after the proper convex glasses are worn, the eyes become 
accustomed to the altered relation between the accommoda¬ 
tion and convergence, and the use of the glasses is attended 
with no discomfort. But when the spherical convex glasses 
are uncomfortable and the eyes cannot get accustomed to 
their use for any length of time, and there is so much com¬ 
plaint from the patient that the optician is compelled to look 
for some remedy, then the prismatic glasses may be tried, and 
an effort made to bring the accommodation and convergence 
into harmony again. Sometimes, in such cases, a pair of 
orthoscopic spectacles will enable the person to read with the 
greatest comfort; but, of course, the book must be held just at 
the focal length of the prismatic combination, in which case 
there is an absence of all effort of either accommodation or 
convergence, and the removal of all the strain which previously 
attended and followed the use of ordinary spectacles. 

On the other hand, it unfortunately happens, in some 
cases, that the orthoscopic glasses not only fail to afford relief, 
but even increase the discomfort that was formerly experi¬ 
enced. As these cases cannot be properly classified as simple 
presbyopia-, the further consideration of their treatment will be 
postponed for the present and taken up again under the head 
of asthenopia. 

There is one point that should be constantly borne in 
min<J, and that is, a rapid increase in the presbyopia, requiring 
freqpent changing of glasses for stronger ones, is the principal 
premonitory symptom of glaucoma; hence, if there is any sus¬ 
picion of this disease, the optician must make careful examina¬ 
tion for it according to the indications laid down in the earlier 
part of this chapter. 

CLOSING REMARKS. 

In closing this chapter on presbyopia, we will repeat a few 
of the important points. You should, in the first place, in all 


PRESBYOPIA. 


405 


cases, ascertain the actual condition of the refraction. Find 
out if there be any existing error, either hypermetropia, my¬ 
opia, or any of the various forms of astigmatism, and correct 
this carefully with a suitable glass or combination of glasses. 

Then, with these glasses in the trial-frame placed before 
the eyes, you measure the range of accommodation to deter¬ 
mine the near point and far point for reading easily the test- 
card that is held in the hand. If you find that the near point 
has receded from the eyes to a point beyond eight inches, you 
add to the glasses in the trial-frame a convex spherical glass 
of sufficient strength to bring the near point back to eight 
inches, and then calculate what the sum of the two glasses 
will be. 

You can thus see that the hypermetrope will always re¬ 
quire glasses stronger than the emmetrope to correct his pres¬ 
byopia, while the myope will require glasses weaker in propor¬ 
tion as his degree of myopia is greater. 

From what has already been said on these subjects, you 
will be able, in every case, to ascertain the refraction and the 
amplitude of accommodation, and the number of the glass 
which an eye requires for vision at any distance. You should 
take the precaution of giving the weaker numbers of convex 
glasses to those yet young and unaccustomed to wearing 
glasses, while you can give a half a dioptre more convex to an 
aged person, wffiose amplitude of accommodation is feeble 
and null. 

You will sometimes meet with cases in which there may 
be amblyopia, and the patient is unable to read the fine type 
with any glass. With such persons, you must ascertain the 
smallest type which they can read, and, using that as your test, 
give them that convex glass which affords the best vision at 
the proper distance. 

At other times the patient may be so illiterate that he 
cannot read; and we sometimes meet with such cases, even in 
this enlightened day. Here you must ascertain for what kind 
of work the glasses are desired, and prescribe that convex 
glass with which he can see to do that work most clearly and 
satisfactorily. 

Perhaps some of our readers may think this chapter is too 


406 


PRESBYOPIA. 


long, and that we have given too much time to the considera¬ 
tion of the accommodation and the proper correction of pres¬ 
byopia. If so, our reply would be to call attention to the great 
importance of near vision in all civilized countries, and of the 
real and practical value of being able to take an exact account 
of the amplitude of the accommodation of the person exam¬ 
ined, and thus to prescribe glasses scientifically. 

TO THE OPTICIAN. 

A large proportion of the persons you will be called upon 
to fit with glasses will be presbyopic, and, consequently, it 
behooves you to acquire a thorough knowledge and a clear 
understanding of this common defect, which, sooner or later, 
affects the eyes of every individual. 

If you are able to fit presbyopia scientifically and satis¬ 
factorily, you will speedily gain an enviable reputation and lay 
the foundation of a growing optical business that will attract 
an increasing class of patients suffering from the more compli¬ 
cated optical defects. It is important for you to understand 
the changes that take place in the eye with the advance of age, 
and especially the changes in the lens and ciliary muscle which 
constitute the condition of presbyopia, with its accompanying 
deficiency of sight; and then your knowledge of the previous 
chapters will indicate the remedy and elucidate the scientific 
principles involved iin the adjustment of the lenses required. 

Presbyopia may seem like a simple defect that requires 
but little care in its correction; but whatever is worth doing at 
all is worth doing well. Remember, little things count, and a 
reputation can be made or marred by the manner in which 
even the simplest duty iis performed. 


APPENDIX. 


Optical Symbols and Abbreviations. 


Ac. 

. Accommodation. 

Aet. 

• • Age. 

Am. 

.. Ametropia. 

An. 

. . Anisometropia. 

As. 

. .Astigmatism. 

Asth. 

. . Asthenopia. 

Ax. 

. . Axis. 

Cc. or — (minus). 

, . .Concave. 

Ce. 

.. Centigrade. 

Cm. 

. . Centimeter. 

Cx. or + (plus). 

.. Convex. 

Cvl. 

. . Cvlinder. 

D. 

. . .Diopter. 

D. Cc. 

. .Double Concave. 

D. Cx. 

. .Double Convex. 

D. T. 

. .Distance Test. 

E. or Em. 

. .Emmetropia. 

H. or Hy. 

. .Hypermetropia. 

In. 

. . Inches. 

L. or L. E. 

. .Left Eye. 

M. or My. 

. .Myopia. 

Mm. 

. .Millimeter. 

N. 

. .Nasal. 

Nv. 

, ..Naked Vision. 

O. D. (Oculus Dexter).. 

. .Right Eye. 

O. S. (Oculus Sinister). .. 

. . Left Eye. 

O. U. (Oculi Unati). 

..Both Eyes. 

P. or Pb.. 

. .Presbyopia. 

P. Cc.. 

. . Periscopic Concave. 

P. Cx. 

.. Periscopic Convex. 

P. D.. 

.. Inter-Pupillary Distance. 


407 

































408 


APPENDIX. 


PI.Plano ; i 

p. p. (Punctum Proximum) Near Point, 
p. r. (Punctum Remotum). Far Point. 

Pr.Prism. 

R. or R. E.Right Eye. 

R. T.Reading Test. 

Rx.Prescription. |- 

Sb.Strabismus. 

S. or Sph.Spherical. 

T.Temporal. 

Ty.Type. 

V.Vision. 

Va.Visual Acuteness. 

W. P.. Working Point. 

+.Plus—Convex. 

—.Minus—Concave. 

3 .Combined with. 

L.At Right Angles. 

°..Degree. 

A.Prism-Diopter. 

..Foot and Minute. 

".. Inch and Second. 

.Line, the twelfth part of an inch 

=.Equal to. 

co.Infinity, 20 feet or further. 



























Glossary of Optical Terms. 


Achromatopsia . . . Color blindness. 

Amblyopia.Impairment or loss of vision without ap 

parent local cause or anomaly to ac¬ 
count for it. Defective sensibility of 
the retina. 


Amblyopia, Subdivisions of: 


Congenital . . . 

. Existing from birth. 

Exanopsia . . . 

. Eoss of sight from 
continued disease of 
the eye. 

Hemeralopia . . 

. Night blindness, due 
to blood degenera¬ 
tion, as in scurvy. 

Hysterical . . , 

. . Effect of nervous re¬ 
flex. A sympa¬ 
thetic condition. 

Nyctalopia . . . 

. Day blindness, due to 
same cause as Hem¬ 


eralopia. 

Toxic. 

. Due to poison ab- 
sorbed into the sys¬ 
tem, as nicotine, 


alcohol, etc. 


Traumatic .... Produced by a blow. 
Ametropia ..... The condition when parallel rays of light 
will not focus in the eye. Opposite of 
‘ Emmetropia or normal vision. 

Anisometropia . . . The condition when each eye has different 
refracting power, necessitating two dif- 
* ferent lenses for correction. 

409 






410 


GLOSSARY. 


Anaesthesia 
Retina . 
Aphakia . 


f f the 


.... Insensibility of the retina. 

.... The condition when the crystalline lens is 
removed, as after operation for cataract. 

Asthenopia.Weak sight, due to weakness of muscles 

controlling the eye. 

Astigmatism .... The condition when, by malformation of 
the cornea or of the crystalline lens or 
other cause, the rays of light from a 
point will not focus at a point after 
passing through the dioptric media. 

Choroiditis.Inflammation of the choroid. 

Diplopia.Double vision. 

Emmetropia .... Normal vision. 

Epiphora.An undue secretion of tears. 

Hemianopsia .... The loss of vision in one-half of the field. 

Heterophoria .... Abnormal tending of vision lines, due to 
want of balance or coordination of 
ocular muscles. 

Heterophoria, subdivisions of, 

Esophoria .... Tending of the visual 
lines inward. 

Exophoria .... Tending outward. 

Hyperphoria . . Tending of the visual 

line of either eye 
above the other. 

Hyperesophoria . Tending up and in¬ 
ward. 

Hyperexophoria . Tending up and out¬ 
ward. 

Heterotropia .... A deviation or squint. 

Heterotropia, subdivisions of, 


Esotropia . , 
Exotropia . 
Hypertropia 


Hyperesotropia 

Hyperexotropia 


Convergent squint. 

Turning in. 
Divergent squint. 

Turning out. 

The condition when 
one eye deviates 
above the other. 
Deviation up and in. 
Deviation up and out 











GLOSSARY. 


4 II 


Hyperinetropia . . . Far sightedness. 

Hypersesthesia . . . Over-sensitiveness of the retina. 

Iritis.Inflammation of the iris. 

Keratitis.Inflammation of the cornea. 

Megalopsia .... Seeing objects larger than normal. 
Metamorphopsia . . Seeing objects distorted. 

Micropsia.Seeing objects smaller than usual. 

Muscse Volitantes . Floating specks or imperfections in field 

of vision, due to shadow of vitrous cells. 

Mydriasis.Unnatural dilatation of the pupil. 

Myopia.Near sightedness. 

Myosis.Unnatural contraction of the pupil. 

Ophthalmia .... Inflammation of the eye. 

Orthophoria .... Coordination of the visual lines of the 
eyes. Perfect muscular equilibrium in 
both eyes. Opposite of Heterophoria. 

Orthotropia.Normal as relates to squint. Opposite 

of Heterotropia. 

Photophobia .... Dread or intolerance of light. 

Presbyopia.Decreased power of accommodation,some¬ 

what vaguely called Old Sight. 

Ptosis.Inability to lift the upper eyelid. 

Retinitis.Inflammation of the retina. 

Strabismus.Squinting of the eyes. 























. 

* 







I 









, • *n 











' 











/ 








, . . . 











































* 






*• 

































* * , i • . .* 





• 
































■ 
























































INDEX 


-A- 

Aberration . 78 

Chromatic . 78 

Spherical . 83 

Accommodation, 196, 221, 

245, 247, 250, 251, 253, 

255, 257, 298, 300, 323, 

329, 372 , 399 


Amplitude of . 290 

Loss of, in Presbyopia.. 332 

Mechanism of .. 120 

Muscle of . 324 

Proportion of Available 

for Use .332, 338 

Range of .123, 290 

Achromatic Lens .. 81 

Acuteness of Vision, 199, 200, 

205, 210, 211, 217, 287, 307 

Amaurosis . 184 

Amber Lenses . 60 

Amblyopia . 184 

Ametropia ......-.no, 197 

Anatomy of the Eye. 17 

Angle of Incidence . 41 

Angle of Refraction ... 41 

Anisometropia .186, 226 

Aqueous Humor ..24, 104 

Asthenopia ... 146 

Astigmatism ...in, 113, 146, 

194, 223, 241 

Compound Hyperme¬ 
tropic . 394 

Compound Myopic . 395 

Mixed . 396 

Simple Hypermetropic.. 391 

“ Myopic . 392 

Test for .113, 289 

With Presbyopia . 391 

Atropia .22, 117 

Atropine, Action of.234, 235 

Axis of Cylinder . 56 

Principal . 44 

Secondary .............. 44 

4 i 3 


Beam . 

Belladonna . 

Bessemer Spectacles 
Bi-concave Lenses . 
Bi-convex Lenses .. 

Bifocal Glasses . 

Binocular Vision ... 

Blind Spot. 

Of Mariotte _ 

Blue Lenses . 

Books of Reference. 
Bridge of Spectacles 


• • • • 33 

.... 117 

.... 151 

• 52 , 54 
• 52 , 55 
388, 389 


124, 140 
107, 108 
.. .. 214 
.60, 148 


153 

73 


C 

Camera, Comparison with... 102 


Case Book... 171 

Cases, Illustrative.. ..221, 233, 

236, 238, 240 

Cataphoria ...268, 275, 278 

Cataract. 290 

Center, Optical .60, 67 

Change of Glasses .. 398 

Choroid .20, 219 

Chromatic Aberration . 78 

Test . 285 

Ciliary Muscle . 120 

Cocaine . 346 

Colored Glasses .146, 147 

Color Blindness. 138 

Sensations. 137 

Concave Lenses...46, 54, 63, 112 

Mirror. 40 

Conjunctiva . 29 

Examination of. 191 

Conjunctivitis . 343 

Convergence ... 124, 254, 256, 

259, 261, 265, 266, 291 

Convex Lenses.54, 63, 112 

Mirror. 40 

Coquilles.61/62, 147, 148 

Cornea, 19, 104, 191, 192, 290, 354 



































































414 


INDEX. 


Cramer.121, 

Cross Eye. 

Crown Glass . 

Crystalline Lens, 18, 25, 104, 
118, 

Cylindrical Lenses .54, 

ID 

Decentered Lenses .66, 

Degree . 

Diaphragm. 

Diffusion Circles.105, 

Dioptric System. 

Diplopia .. 146, 267, 269, 292, 

Crossed.274, 

Heteronymous.274, 

Homonymous .274, 


Vertical . 

Direct Vision... 109, 182, 216, 

Disease, Organic . 

Dispersion of Light. 

Divergence, Measure of..263, 

Divergent Ray .33, 

Donders .11, 310, 326, 

Dot and Line Test . 

Double-Focus Glass . 

Lenses. 

Sight . 

Vision .124, 

Duboisine. 

ZEJ 

Ellipse . 

Ellipsoid . 

Emerson’s Perimeter. 

Emmetropia ...no, 195, 196, 
197 , 


Errors, Cases 

Illustrating, 


359 , 

Eso-cataphoria . 

.268, 

Esophoria . 

.268, 

Examination of Eyes, Objec- 


tive. 

Examination of Eyes, Subjec¬ 
tive . 

Exo-cataphoria .268, 

Exophoria.268, 269, 274, 

Exophthalmus . 

Extra Fronts. 

Eye-ball. 

Eye-brows . 

Eye-glasses .72, 

Eye-lashes . 

Eye-lids . 

Eye-protector . 


Eyes, Change in from Age.. 308 


Distance Between. 188 

Movements of. 260 

Eye-strain . 343 

IF 1 

Far Point.123, 248, 290 

Sight . 146 

Field of Vision ....109, 135, 212 

Flaws in Lenses. 92 

Focus. 39 

Conjugate . 40 

Negative . 41 

Principal .40, 44 

Virtual . 41 

Frame, Fitting of. 402 

Too Narrow. 403 

Too Wide . 403 

Frameless Lenses. 72 


Ol- 

Glasses Not to Magnify too 

Much . 400 

Glaucoma.218, 344, 355 

Atropine Contraindicated 345 

Diagnosis of. 355 

Recognition of . 356 

Symptoms of. 345 

Goggles. 61 

Graduation on Trial-frame... 163 

Green and Monoyer. 168 

Green Lenses . 60 

X3C 

Halo, Appearance of. 352 

Helmholtz .121, 129, 326 

Hemorrhages, Retinal.218 

Heterophoria .268, 273 

Homatropine.346 

Horizontal Meridiah. 163 

Humor, Aqueous . 24 

Crystalline . 25 

Vitreous . 25 

Hyaline Membrane. 25 

Hyper-esophoria.268, 280 

Hyper-exophoria .268, 280 

Hypermetropia, in, 112, 146, 

186, 193, 199, 207, 209, 

219, 221, 300, 301, 302, 

322, 326, 327, 331, 387 

Test for. 289 

With Presbyopia ... .312, 

322, 327, 331, 386 
Hyperphoria .268, 275, 277 


326 

3 i 

53 

309 

55 

373 

202 

85 

112 

96 

293 

277 

2 77 

2 77 

281 

219 

290 

79 

292 

ii 5 

383 

270 

75 

52 

146 

140 

346 

318 

318 

213 

301 

360 

279 

269 

253 

253 

280 

2 77 

190 

75 

17 

27 

75 

29 

27 

135 
















































































INDEX. 


415 


X 

Illuminated Bodies. 

Illumination .201, 

Oblique . 

Illustrative Cases ...221, 233, 
236, 238, 

Images, Negative. 

Real . 

Vertical . 

Inch System of Numbering. . 
Indirect Vision, 143, 182, 216, 
Instruction in Optics, Value 

of . 

Insufficiency of Muscles, 267, 

Inverted Image. 

Iris.18, 

Examination of. 

Irregular Sight. 

T 

Jaeger’s Test-letters . 

Josh Billings ... 


Keratometer . 152 , 

Keratoscope . 152 , 

Xj 

Lachrymal Apparatus. 

Landolt .334 


Lens, Crystalline, 18, 25, 104, 
118, 

Achromatic . 

Amber ... 

Bi-concave .S 2 , 

Blue .. 

Compound Cylindrical .. 

Concave.46, 54 , 63, 

Convex . 54 63, 

Cylindrical . 54 55 , 

Decentered. 

Double. 

Frameless . 

Green ... 

Measurement of. 


Meniscus .47, 

Numbering of. 

Orthoscopic .59, 373, 

Pebble .. 

Periscopic.47, 48 , 


Plano-concave 
Plano-convex 
Prismatic .... 
Red. 


Lens, Smoke .. . 60 

Spherical. 159 

Tinted . 60 

Lens-Holder . 161 

Lens-Measure . 89 

Lenticular Glasses. 75 

Light .. 33 

Reflection of. 37 

London-smoke Glasses ...61, 150 
Lorgnettes . 72 


Maddox’s Double Prism ... 284 


Magic Lantern . 196 

Measurement of Lenses. 86 

Measuring Stick.152, 170 

Media, Refracting . 104 

Medical Profession, Jealousy 

of . 358 

Meibomian Glands . 28 

Meniscus Lenses .47, 48 

Meter Angle . 255 

Method of Examination .... 183 

Metric Rule .152, 170 

Metrology of the Eye. 183 

Mica Spectacles. 61 

Minute . 202 

Mirror, Concave . 40 

Convex . 40 

Monocle . 72 

Movements of Eyes.260 

Muscle, Ciliary. 120 

Inferior Oblique.283 

Superior Oblique. 283 

Muscles, Oblique, Insuffi¬ 
ciency of.281, 283 

Test, with Prisms. 261 

Weak, Detection of.227 


Muscles of Eye, 31, 263, 265, 292 
Muscular Insufficiency . .267, 292 
Myopia .-..in, 113, 146, 186, 

193, 199, 207, 209, 220, 

239, 300, 302, 313, 322, 

326, 327, 331, 389 

Test for. 289 

With Presbyopia ... .313, 

322, 326, 331, 389 

XT 

Near Point, 122, 248, 290, 297, 

301, 302, 325, 328, 367, 378 


Near Sight . 146 

Near Vision, Testing. 398 

Negative Images . 142 

Nerve, Optic . 103 


33 

210 

192 

240 

61 

5 i 

5 i 

95 

217 

363 

292 

132 

21 

193 

146 

168 

169 

179 

179 

30 

335 

309 

81 

60 

54 

60 

55 

112 

112 

159 

66 

83 

72 

60 

86 

48 

03 

404 

53 

52 

54 

54 

54 

61 














































































INDEX. 


416 

Neuralgia, Ciliary.. 352 

Neutralization .. 87 

Nodal Point..44, 167 

O 

Objective Examination .252 

Oculist and Optician. 362 

Old Sight. 146 

Opaque Bodies .... .. 35 

Ophthalmometer .152, 178 

Ophthalmoscope ...130, 152, 

173, 174, 227, 290, 362 
Ophthalmoscope, As a Meth¬ 
od of Examination.... 227 
Ophthalmoscope, Examina¬ 
tion by. 228 

Optical Bracket.152, 181 

Center .60, 67 

Optic Disk, Cupping of .... 354 

Optician, Definition of . 183 

And Oculist. 327 

Personal Remarks to ... 406 
Province of, Enlarged, 

358 , 363 

Optic Nerve . 103 

Optics . 33 

Optical Education .. 152 

Orbit . 26 

Orthophoria .. .267, 273, 275, 277 
Orthoscopic Lenses .59, 373, 404 
Outfit Required . 152 


IF 

Pantoscopic Glasses . 75 

Parabolic .. 83 

Parallel Rays .34, 115 

Pebble Lenses. 53 

Testers .. 53 

Pencil . 33 

Perception of Colors.217 

Of Light .217, 222 

Perimeter .152, 182 

Emerson’s ... 213 

Periscopic Lenses ....47, 48, 52 

Periscopism ..86,319 

Phorometer .152, 180 

Physiology of Vision. 102 

Pilocarpine, Use of . 325 

Pin-hole Disk.159, 288 

Test . 194 

Pray’s Letters . 169 

Presbyopia, 77, 123, 146, 193, 

295-406 

And Hypermetropia, 312, 

322, 326, 327, 331, 386 


Presbyopia and Myopia, 312, 

322, 326, 327, 331, 389 

Appearance of. 334 

Commencement of.313 

Complications of. 385 

Definition of ...315, 326, 

334 , 335 

Degrees of, at Different 

Ages . 336 

Diagnosis of.369, 383 

Glasses for Day and 

Night Use . 382 

Glasses, Not too Strong. 375 

Grade of . 370 

Manifestations of.341 

Method of Examination. 381 
Number of First Glass... 372 
Principle Involved in 

Correction of . 367 

Rule for Correction of... 378 

Tables for . 384 

Time of Life it Occurs.. 333 

Treatment of. 365 

With Astigmatism . 391 

Prejudice Against Glasses... 371 
Pride Prevents Glasses from 

Being- Worn . 371 

Prisms, ..42, 58, 69, 124, 125, 

159, 261 

Prisoptometer .152, 177 

Protrusion of Eye-ball. 190 

Pupil .21, 121, 235, 307, 352 

Pupillary Distance. 188 

Pupillometers . 190 


IF, 

Radiating Lines .... . 170 

Rainbow Appearance.. 352 

Ray .... 33 

Rays, Divergent.33, 115 

Parallel .34, H 5 

Reading, Distance to be 

Held . 339 

Recapitulation of Distant 

Vision Tests ......... 244 

Of Method of Examina¬ 
tion . 287 

Record Book.152, 171 

Red Lenses . 61 

Reflection, Angle of. 39 

From Curved Surfaces.. 39 

Of Light ... 37 

Refracting Media. 104 

Refraction, no, in, 112, 199, 

200, 301 

Of Light ...:. 72 





































































INDEX. 


417 


Retina .19, 23, 106 

Sensibility of .141, 143 

Retinoscope .152, 175 

Retinoscopy. 232 


Scheffler.• 373 

Sclerotic . 18 

Scotoma. 218 

Second . 202 

Shadow Test. 176 

Single Vision with Two Eyes 57 

Sleep .:. 144 

Smoke Lenses.60, 149 

Snellen’s Test-letters-167, 


184, 

202, 207 

Spectacles. 

.... 72 

First Use of. 

..•• 13 

Spectrum . 

.... 79 

Spherical Aberration ... 

.... 83 

Split Glasses . 

.... 389 

Spot, Blind. 

.107, 108 

Yellow. 


Spy-glass . 

.... 114 

Stenopaic Slit . 

.... 159 

Stereopticon. 

.... 171 

Stereoscope . 

.... 126 

Strabismus . 

.... 65 

Subjective Examination 

. 185 

Sunlight . 



Tears . 30 

Temples of Spectacles . 74 


Tension of Eye-ball _347, 350 

Tension Signs . 349 

Test-card, Distance of. .167, 168 
Test-lenses, 152, 153, 154, 155, 156 

Test-letters .147, 148 

Test-types.152, 165, 168, 206 

Tinted Lenses .60, 146 

Tonometers .'. 348 

Translucent Bodies. 35 

Transparent Bodies . 35 

Trial-frame. 162 

Turner’s Spectacles . 150 

■v 

Vertical Meridian . 162 

Vision, Conditions Necessary 

for. 103 

Binocular .124, 140 

Direct ....109, 182, 216, 219 

Double.124, 140 

Field of. 109 

Indirect . .. 143, 182, 216, 217 

Periodic Dimness of. 353 

Visual Angle . 204 

Vitreous Humor .19, 25, 104, 353 

■w 

Weak Sight. 146 

Wild Hairs . 29 

■sr 

Yellow Spot. 106 

Young, Thomas. 184 

















































NOTICE TO READERS. 


The Optician's Manual is a republication in book form of 
the serial The Optician's Manual published monthly in The 
Keystone , from May, 1890, to November, 1896, inclusive. 
While the serial, as published with much new matter and 
illustrations in this second edition, covers the science of practical 
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in order to make it, when complete, not only the standard 
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and comprehensive work on each subdivision of the science. 
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and Muscle Test at greater length and in minutest detail, to 
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several years will elapse before the continuation of the serial 
will be complete and procurable in book form. For this and 
other reasons a perusal each month of the Optical Department 
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OPTOMETRIC RECORD-BOOK. 


A record-book, wherein to record optometric examina¬ 
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The Keystone Optometric Record-book was specially 
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only a record-book, but an invaluable guide in examination. 

The book contains two hundred record forms with printed 
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Each book has an index, which enables the optician to 
refer instantly to the case of any particular patient. 

The Keystone Record-book diminishes the time and 
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